Abstract:
Switching apparatus is used in combination with a multiplicity of mass storage units to provide a user of a digital computer with privacy from other local users and from users on a connected network. When the computer is connected to the network, the private files are protected from computer viruses, worms, and other pieces of destructive code. When the computer is not connected to a network, various local users can maintain their own programs and data files in complete privacy from other local users and safe from any harm that may have been intended by malicious action directed at the computer from the network. Special-purpose computers and other digital systems can also be protected by the use of such a switched mass memory system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to the selective isolation of a first set of mass storage units in a digital computer or other digital system from a second set of mass storage units in the same digital computer or other digital system so that information transfer between mass storage units in the first set and mass storage units in the second set is prevented. Thus, when one set of mass storage units is made active, a second set of mass storage units is made inactive or disabled. For example, the first set may consist of a single hard disk and the second set may consist also of a single hard disk. In another embodiment of this invention, for example, the first set again consists of a single hard disk, but the second set may consist of several subsets. Each of those subsets consists of a single hard disk. The disks in the second set are interconnected in such a way that any member of the second set can be enabled with the result that that disk becomes a new first set while the disk that formerly constituted the first set is relegated to a newly formed second set, replacing the newly enabled disk therein. In other embodiments the first set, as well as the second set, may comprise more than one member.  
         [0003]     2. Description of the Prior Art  
         [0004]     The use of multiple disks in a digital computer is not new. Multiple disks have been used to increase the storage capacity of a computer beyond what is possible with a single disk. Dual booting of digital computers has been employed to permit selection of a particular hard disk or partition from which to boot an operating system in computers in which operating systems have been installed on multiple hard disks and/or partitions. The selection has been made by software means, and all hard disks and/or partitions have been accessible by software after the operating system has been booted. Thus, file transfer among all of the hard disks/partitions in the computer has been possible after the booting operation has been completed.  
         [0005]     This capability to access all files stored in the computer, regardless of which disk contains the operating system that was booted, has been considered important because it allows storage of files on any desired disk or partition, to implement a desired filing system, for example, without loss of access to any file in the system.  
         [0006]     Multiple disks have been used also to increase the reliability of a computer, by providing multiple copies of the information stored in the computer on separate disks. In such a system, if one disk should fail, the stored data can be retrieved from another disk.  
         [0007]     RAID (Redundant Array of Inexpensive Disks) systems have been used to enhance performance in a number of ways. Disk striping, a process of distributing data reads and writes across multiple disks, reduces the effect of head seek time on speed of data transfer. Disk mirroring and duplexing provide protection against loss of data by writing duplicate data to different disks. Error correcting code in a RAID provides some protection against data loss by storing a check sum on the disk.  
         [0008]     In all of these previous multiple disk systems, all disks are or can be enabled concurrently, and data and programs stored on one disk can be transferred to another disk. Consequently, a destructive program or piece of code that is admitted to one disk can contaminate all disks in the system.  
         [0009]     A purpose of this invention is to provide isolation of one or more mass storage units, such as hard disks, from another mass storage unit or group of mass storage units, to ensure privacy of data and programs and to protect against hacking and other harmful or destructive attacks directed at a digital system.  
         [0010]     External hard disks have been used to provide increased storage capacity and portability of files. Such hard disks do not, however, provide the isolation made available with this invention, because the external hard disks heretofore available can be independently enabled and thus allow for transfer of data and program code among them and between them and internal hard disks.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The essence of the preferred embodiment of this invention is a system for selecting and enabling one or more of a multiplicity of mass storage units for operation at any given time, while disabling others or ensuring that those others are not enabled. That is, those mass storage units that were previously enabled, other than any of the newly selected mass storage units that are in that group, are disabled, and those mass storage units other than the newly selected mass storage units that were previously not enabled are prevented from being enabled. The selecting and enabling operations are performed by a switching apparatus that comprises selecting or identifying apparatus and enabling apparatus. At any given time each one of the multiplicity of mass storage units has an enabled status, which may be either enabled or not enabled, and that enabled status is determined by the switching apparatus.  
         [0012]     The selection of one or more of the multiplicity of mass storage units for operation at any given time may be made by hardware or by firmware or software. One of the mass storage units may be a primary mass storage unit, regarded as a part of a computer itself, while the remaining mass storage units collectively are a part of the apparatus disclosed by this invention. Alternatively, all of the mass storage units collectively may be a part of the apparatus disclosed by this invention.  
         [0013]     In another form, the invention comprises the selection and enabling apparatus, but not the mass storage units. Again, the enabling apparatus is capable of enabling one or more of a multiplicity of mass storage units for operation at any given time, while disabling others or ensuring that those others are not enabled, provided that the mass storage units are added.  
         [0014]     In some embodiments, only one mass storage unit is made operational; the other mass storage units in the system are disabled or not enabled. Consequently, it is not possible to exchange files among the various mass storage units, and each mass storage unit defines a distinct digital computer, on the basis of the programs and data stored within it. In effect, multiple digital computers are made available within what appears to be a single digital computer, by the selection of the mass storage unit to be used. Each mass storage unit may employ a distinct operating system, or the same operating system may be used on two or more of the mass storage units.  
         [0015]     This invention encompasses all types of mass storage units, regardless of the kind of interface with the rest of the computer, and all kinds of digital systems, special-purpose systems as well as general-purpose digital computers. The interface may be IDE, SCSI, parallel port, USB, Firewire, wireless, optical, or any other kind The digital system may be a mainframe, a personal computer (IBM, IBM-compatible, or Macintosh, for example), or any other kind, including a reservation system and a multifunction telephone, among others.  
         [0016]     A more complex system that falls also within the scope of this invention is a system comprising more than two mass storage units, in which any selected combination of those mass storage units can be enabled and the remaining mass storage units disabled or not enabled.  
         [0017]     As an example of a simple embodiment, a digital computer can be provided with multiple hard disks. A particular one of the multiple hard disks can be selected by a switching system that may be mechanical, optical, electrical, software, firmware, or some combination thereof; the other hard disks are maintained in an inoperative state. If the switching system is so constructed that a change in the selection of the active disk can be achieved only by use of a distinct key or code for each selection, then each disk can be assigned to a different user, and each user can maintain his files in complete privacy from the other users. The locking device in which the key or code is used may be hardware, software, firmware, or a combination thereof.  
         [0018]     As another example, a given computer can be provided with two hard disks. A particular one of the two hard disks can be selected by a switching system that may be mechanical, optical, electrical, software, firmware, or some combination thereof; the other hard disk is maintained in an inoperative state. Thus, it is possible to operate one “computer” offline at times to maintain privacy of data files from a connected network or alternatively operate the other “computer” in the network at other times to allow exchange of information with other computers via the network.  
         [0019]     In addition to maintaining privacy of all files on the first “computer” from other users of the network, this system protects the first “computer” from viruses, worms, and all other forms of harmful intrusion transmitted over the network while still allowing uninhibited use of the network on the second “computer”. If disaster strikes, in the form of a virus attack, for example, all of the files on the private “computer” are unaffected. Software on the hard disk that defines the public “computer” can be restored without endangering the private files on the other hard disk, and operation can be resumed with minimal trauma. Only the programs and other files to be used on the network will be kept on the public disk, so only they will need to be restored after disaster strikes. If only a minimal set of programs and other files are stored on the public disk, the effort required to recover from the disaster is minimized.  
         [0020]     Even if antivirus software is used, viruses and other harmful pieces of code can infect a computer, because the user has not kept the antivirus software up to date or simply because protection against a new piece of infectious code has not yet been incorporated into the antivirus software by the supplier. Therefore, the use of an isolated disk system can be of benefit to even those users who employ protective software.  
         [0021]     The same protection can be achieved, of course, by physically removing one hard disk and replacing it with another. Such a process is cumbersome and time consuming, however. Moreover, it introduces the possibility of causing substantial damage to the computer.  
         [0022]     The use of two completely independent conventional computers will provide the same protection against data corruption, but this invention provides the desired capability at a very substantially reduced cost in terms of weight, volume, and dollars.  
         [0023]     Although only one of the mass storage units can be activated or operational at any given time in the preferred embodiment, in other embodiments there is no such restriction. In some such embodiments, a single mass storage unit or other proper subset of the totality of mass storage units in the system is activated when power is made available, as described above; but after the computer is in operation, hardware and/or firmware or software can be used to enable one or more other mass storage units, so that data can be exchanged freely among the various units.  
         [0024]     Thus, this invention discloses also a hardware multiple boot system, which is more convenient to install and more convenient to operate than existing software multiple boot systems.  
         [0025]     Also disclosed by this invention is a switchable mass memory system comprising a group of mass storage units for use with a separate mass storage unit that is a part of another digital system, a switching apparatus for selecting one of the totality of mass storage units, and an enabling apparatus for enabling the selected mass storage unit and ensuring that the other mass storage units are not enabled. The separate mass storage unit may, for example, be the original hard disk in a digital computer, while the switchable mass storage system is an add-on system or upgrade to the digital computer.  
         [0026]     To facilitate installation of the switching and enabling system disclosed in this invention in a personal computer, the switching and enabling system can be provided with one or more connectors appropriate for mating with standard connectors provided within a personal computer.  
         [0027]     The above and other advantages and features of the invention will be apparent to those skilled in the art from the following descriptions of particular embodiments taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a block diagram depicting the major functional units of a switchable mass storage system.  
         [0029]      FIG. 2  illustrates one embodiment of this invention, in which solid-state switching of two hard disks is utilized, with protection against inadvertent switching of the disks while power is applied.  
         [0030]      FIG. 3  illustrates an embodiment of this invention in which the number of disks from which selection can be made is greater than two, with protection against inadvertent switching of disks while power is applied.  
         [0031]      FIG. 4  illustrates the use of a connector to facilitate the connection of the switching apparatus to the power supply in a digital computer.  
         [0032]      FIG. 5  depicts one form of lock that can be used to prevent unauthorized activation of mass storage units. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The block diagram in  FIG. 1  depicts in a very general way a switchable mass storage system encompassed by this invention. A multiplicity of mass storage units (MSUs)  105  is connected via a first connecting apparatus  107  to enabling apparatus  103  capable of enabling one or more of the multiplicity of MSUs  105  and disabling the remainder of the multiplicity of MSUs  105  or ensuring that they are not enabled. For simplicity of expression, terms such as “disabling the remainder of” and “disable the remainder of” are used hereafter to include “ensuring that they are not enabled” and “ensure that they are not enabled”, respectively, unless the context clearly implies otherwise. Also for simplicity of expression, terms such as “enabling one or more of” or “enable one or more of” are sometimes used hereafter to include disabling other units, unless the context clearly implies otherwise. The particular mass storage units in the multiplicity of MSUs  105  to be enabled are identified by signals provided via a second connecting apparatus  109  to the enabling apparatus  103  by identification or selection apparatus  101 . Each of the first connecting apparatus  107  and the second connecting apparatus  109  may contain a connector to facilitate connection. The multiplicity of mass storage units  105 , the first connecting apparatus  107 , the enabling apparatus  103 , the second connecting apparatus  109 , and the identification or selection apparatus  101  may be a part of a digital computer, having been installed therein when the digital computer was constructed. In some embodiments of this invention, the second connecting apparatus  109  is absent, because the selection apparatus  101  and the enabling apparatus  103  are the same or are integrated into a combined switching apparatus.  
         [0034]     Also encompassed by this invention is a kit comprising the enabling apparatus  103 , the first connecting apparatus  107 , and one or more mass storage units exclusive of a mass storage unit, regarded as a primary mass storage unit, contained within an existing digital computer to which the kit is intended to be added. Such a kit may be regarded as an upgrade for a digital computer. Some such kits comprise also selection apparatus  101  distinct from the enabling apparatus  103 , as well as the second connecting apparatus  109 .  
         [0035]     Another structure encompassed by this invention comprises the enabling apparatus  103  and the first connecting apparatus  107 , but no mass storage unit. Such a structure may also take the form of a kit for upgrading a digital computer, but it may be used with a multiplicity of mass storage units  105 , regardless of whether any of the mass storage units is contained within a digital computer. Some such kits comprise also selection apparatus  101  distinct from the enabling apparatus  103 , as well as the second connecting apparatus  109 .  
         [0036]     Although this invention can be implemented by switching one or more control signals (in a set comprising ready, chip select, and drive active, among others, in an IDE interface cable, for example) to the mass storage units, in the preferred implementation it is the power connections that are switched. The mass storage units encompassed by this invention may be but need not be associated with a general-purpose digital computer.  
         [0037]     In an IBM personal computer or equivalent, for example, the modification required to switch the power connections to two internal hard disk drives with IDE interfaces serving as MSUs is much simpler to implement than a modification in the control lines. This invention encompasses all forms of MSU, however, including external devices that may be connected to the computer by serial port, parallel port, universal serial bus, Firewire, and any other form of information transfer apparatus.  
         [0038]     One embodiment of a switched mass storage system comprises two internal hard disk drives in an IBM PC or an IBM-compatible PC with IDE interface, for example. In this example, the computer originally contained a single hard disk. A switched mass storage system is added, to form an isolated disk system. The added switched mass storage system is an assembly comprising a male connector that mates with one of the female connectors provided in the computer for supplying power to hard disk drives; two female connectors identical to the ones provided in the computer for supplying power to hard disk drives; the additional hard disk drive that is to serve as the isolated disk; and a switching circuit. The +5 volt pin and the +12 volt pin in the male connector are connected as inputs to the switching circuit. The outputs of the switching circuit include a +5 volt wire and a +12 volt wire connected to the appropriate pins in one of the female connectors in the assembly, and a +5 volt wire and a +12 volt wire connected to the appropriate pins in the other female connector in the assembly. The ground pins in the male connector are connected to the ground pins in each of the female connectors, either directly or through the switching circuit The switching circuit is a combination of the selection apparatus  101 , the enabling apparatus  103 , and the second connecting apparatus  109 .  
         [0039]     Prior to installation of the isolated disk assembly in the computer, the hard disk in the assembly, which is to be added to the computer, is formatted in the usual way and the desired operating system is installed. Installation of the isolated disk system in the computer consists of a) configuring the hard disk drive in the assembly as a master, b) physically installing the hard disk drive in the assembly in the computer, c) connecting the hard disk drive in the assembly to a hard disk controller cable, using the connectors provided in the computer for that purpose; d) physically installing the structure holding the switching circuit (typically a printed-circuit board mounted on a metal bracket that can be substituted for a cover on one of the slots on the back of the computer, with a switch mounted on the bracket so that it can be actuated from outside the computer); e) connecting the male connector in the isolated disk assembly to one of the female connectors provided in the computer for supplying power to hard disk drives; f) connecting one of the female connectors in the isolated disk assembly to the hard disk drive in the assembly; and g) replacing the female power connector that was originally connected to the original hard disk drive in the computer with the second female connector in the isolated disk assembly. The hard disk in the assembly is then selected, and the computer is booted and an operating system is installed in the usual way.  
         [0040]     Thereafter, either hard disk can be selected prior to booting the computer, and the computer will function normally with the selected hard disk serving as the mass storage unit of the computer.  
         [0041]     A minor modification of this embodiment is the incorporation of the switch of the isolated disk system in the power switch of the computer so that the power switch has multiple positions: off, on with only the first of the hard drives active, and on with only the second of the hard drives active.  
         [0042]     If one of the hard disk drives is configured as a master with slave and the other hard disk drive is configured as a slave, then after the computer has been booted with the master drive, power can be applied to the second drive with a modified form of the switching circuit, so that both hard disk drives can be operated concurrently, under the control of software, in systems in which operation with two operating systems installed is permitted. This would not be done, of course, in a system in which disk isolation is desired.  
         [0043]     A very simple form of switching circuit for the isolated drive system comprises a two-pole, double-throw switch. One of the rotors of the switch is connected to the +5 volt supply, and the other rotor is connected to the +12 volt supply. In a first position of the rotors, the stator terminal in contact with the +5 volt rotor is connected to the +5 volt terminal of a first hard disk drive, and the stator terminal in contact with the +12 volt rotor is connected to the +12 volt terminal of that same hard disk drive. In the second position of the rotors of the switch, the stator terminals in contact with the +5 volt rotor and the +12 volt rotor are connected to the +5 volt terminal and the +12 volt terminal, respectively, of a second hard disk drive. In this way, the enabled state of the first hard disk drive and the enabled state of the second hard disk drive are determined by the switching apparatus.  
         [0044]     Simplicity and economy are advantages of this kind of switching apparatus; the selection apparatus is the switch, and the enabling apparatus is the same switch.  
         [0045]     A disadvantage of this kind of switching circuit is that the switch can be actuated inadvertently while the computer is in operation, which can result in loss of data and malfunction of the software. Clearly, an improved embodiment would inhibit switching except at the time the computer is booted. Such inhibition of switching can be achieved by inhibiting changes in the identification or selection of mass storage units to be enabled except at the time the computer is booted or by inhibiting changes in the disabling/enabling of mass storage units except at the time the computer is booted, regardless of whether changes in identification or selection have been made.  
         [0046]     One embodiment of a switching circuit that incorporates such inhibition of switching incorporates a power-on reset kind of circuit (comprising a resistor-capacitor charging circuit or a 555 timer circuit, for example), a single-pole, single-throw switch, and a relay with a holding contact. When the computer is booted, the +12 volt supply is connected for only a short interval of time (the power-on delay time) to the rotor of the single-pole, single-throw switch. The stator terminal of the switch is connected to one terminal of the relay coil; the other terminal of the relay coil is grounded. The relay has three sets of double-throw contacts (i.e., form C). Two of the three sets of contacts are connected as the terminals on the double-pole, double-throw switch in the previous example. The third set of contacts on the relay is used as a holding circuit.  
         [0047]     If the switch was open, and hence the first disk drive was selected, at the time the computer was booted, the relay is not actuated at boot-up; and it cannot be actuated after the power-on delay time has expired because the voltage applied to the rotor of the single-pole, single-throw switch is then zero. Therefore, the selection of disk drive to be used cannot be changed in that event.  
         [0048]     If the switch was closed, and hence the second disk drive was selected, at the time the computer was booted, the relay is actuated at boot-up. The holding contacts then serve to maintain the connection from the +12V supply to the relay coil after the power-on delay time has expired. Because of the action of the holding circuit, the selection of disk drive to be used cannot be changed later by changing the state of the switch in that event, either.  
         [0049]     Thus, changes in the enabled state of the mass storage units are inhibited except at the time the computer is booted.  
         [0050]     In another, preferred, embodiment the relay of the preceding example is replaced by a solid-state circuit. Such an embodiment is illustrated in the following example, with reference to  FIG. 2 .  
         [0051]     A single-pole, double-throw switch  1  is used instead of a single-pole, single-throw switch because the buffered flip-flop comprised of open-collector NAND gates  17 ,  19 ,  21 , and  23  and the associated pull-up resistors  13 ,  15 ,  77 ,  81 ,  79 , and  83  has no default state, as does the relay in the embodiment described above. The four open-collector NAND gates  17 ,  19 ,  21 , and  23  may collectively be a type 7403 integrated circuit, for example. At the instant that power is applied to the circuit, the capacitor  11  begins to charge through the resistor  9 , providing a transient logic zero at the rotor  3  of the switch  1 . If at that time the rotor  3  is in contact with a first stator  7  of the switch  1 , then the flip-flop is forced into its logic 0 state, with a logic 0 at the output of one primary NAND gate  17  and a logic 1 at the output of its associated buffering NAND gate  21 . The logic 1 voltage at the output of the buffering NAND gate  21  is applied via a conductor  99  through a first voltage divider comprising two resistors  25  and  27  to the base of a first transistor  33  and through a second voltage divider comprising two other resistors  35  and  37  to the base of a second transistor  43 . As a result, the first transistor  33  and the second transistor  43  (each of which may be a 2N2222, for example) are turned on.  
         [0052]     A third transistor  65  is turned on by the voltage drop across its base resistor  31  that results from the collector current in the first transistor  33 , which appears also in the base resistor  31  and in the collector resistor  29  associated with the first transistor  33 . Similarly, a fourth transistor  67  is turned on by the voltage drop across its base resistor  41  that results from the collector current in the second transistor  43 , which appears also in the base resistor  41  and in the collector resistor  39  associated with the second transistor  43 . Thus, Drive A  73  is enabled or powered on by the +12 volts applied to it through the third transistor  65  and the +5 volts applied to it through the fourth transistor  67 . (Each of these power transistors  65  and  67  may be a 2N4920 transistor, for example, but PNP transistors with higher current ratings may be used if Drive A  73  requires more current than the 2N4920 transistor can supply.)  
         [0053]     At the same time, the buffered flip-flop provides a logic 1 at the output of a second primary NAND gate  19  and a logic 0 at the output of its associated buffering NAND gate  23 . Consequently, a fifth transistor  53 , sixth transistor  63 , seventh transistor  69 , and eighth transistor  71  are prevented from conducting collector currents, and thus Drive B  75  is disconnected from the +5 volt supply and the +12 volt supply.  
         [0054]     If, however, at the instant power is applied to the circuit the rotor  3  of the switch  1  is in contact with the second stator  5  of the switch  1 , then the flip-flop is forced into its logic 1 state, with a logic 0 at the output of NAND gate  19  and a logic 1 at the output of the buffering NAND gate  23 . The logic 1 voltage at the output of the buffering NAND gate  23  is applied via a conductor  97  through a third voltage divider comprising two resistors  45  and  47  to the base of the fifth transistor  53  and through a fourth voltage divider comprising two other resistors  55  and  57  to the base of the sixth transistor  63 . As a result, the fifth transistor  53  and the sixth transistor  63 , each of which may be a 2N2222, for example, are turned on.  
         [0055]     The seventh transistor  69  is turned on by the voltage drop across its base resistor  51  that results from the collector current in the fifth transistor  53 , which appears also in the base resistor  51  and in the collector resistor  49  associated with the fifth transistor  53 . Similarly, the eighth transistor  71  is turned on by the voltage drop across its base resistor  61  that results from the collector current in the sixth transistor  63 , which appears also in the base resistor  61  and in the collector resistor  59  associated with the sixth transistor  63 .  
         [0056]     Thus, Drive B  75  is enabled or powered on by the +12 volts applied to it through the seventh transistor  69  and the +5 volts applied to it through the eighth transistor  71 . Each of these power transistors  69  and  71  may be a 2N4920 transistor, for example, but PNP transistors with higher current ratings, such as the 2N6029 transistor, may be used if Drive B  75  requires more current than the 2N4920 transistor can supply.  
         [0057]     At the same time, the buffered flip-flop provides a logic 1 at the output of the first primary NAND gate  17  and a logic 0 at the output of its associated buffering NAND gate  21 . Consequently, the first transistor  33 , the second transistor  43 , the third transistor  65 , and the fourth transistor  67  are turned off, and Drive A  73  is disconnected from the +5 volt supply and the +12 volt supply.  
         [0058]     After the capacitor  11  has charged, the voltage on the rotor  3  of the switch  1 , and hence the voltage on whichever of the stator terminals  5  and  7  is in contact with the rotor  3 , is a logic 1 voltage. If the state of the switch  1  is changed after the capacitor  11  has charged, therefore, the buffered flip-flop does not respond, because the inputs to the first primary NAND gate  17  and the second primary NAND gate  19  are both logic 1. That is, the flip-flop memorizes the state of the switch  1  at the instant that power is applied to the circuit, and changing the position of the switch  1  thereafter has no effect on the enabling of Drive A  73  and Drive B  75 . Thus, changes in the state of the mass storage units are inhibited except during a short interval of time immediately after power is made available. Although the above description of a preferred embodiment of this invention encompasses the mass storage units represented therein by Drive A  73  and Drive B  75 , in other embodiments a switching system, one version of which comprises all components shown in  FIG. 2  except Drive A  73  and Drive B  75 , can be provided alone, with mass storage units obtained from another source.  
         [0059]     Optionally, in such an embodiment the switching system can be provided with one or more connectors such as would be appropriate for mating with a standard power connector for a hard disk drive in a digital computer or a standard power connector on a hard disk drive or both, for example.  
         [0060]     As another example of this invention, a switched disk system comprising more than two disk drives is illustrated in  FIG. 3 . The total number of disk drives is n; the drives are identified as drive  0   58 , drive  1   60 , . . . , and drive n- 1   62 .  
         [0061]     Drive  0   58  is enabled by connecting it to the +5 volt power supply via a supply line  112  and another supply line  128  through a first power switch  46 , and to the +12 volt power supply via a supply line  122  and another supply line  134  through a second power switch  48 . Similarly, drives  1   60 , . . . , n- 1   62  are connected to the +5V power supply via supply lines  114 , . . . ,  116  and other supply lines  130 , . . . ,  132  through power switches  50 , . . . ,  54 , and to the +12V power supply via supply lines  124 , . . . ,  126  and other supply lines  136 , . . . ,  138  through power switches  52 , . . . ,  56 . The first power switch  46  shown in  FIG. 3  may be regarded as having the structure illustrated in  FIG. 2 , comprising the resistors  35 ,  37 ,  39 , and  41 , and the transistors  43  and  67 ; the second power switch  48  shown in  FIG. 3  has the structure illustrated in  FIG. 2  comprising the resistors  25 ,  27 ,  29 , and  31 , and the transistors  33  and  65 . The structure of the power switches  50 ,  52 , . . .  54 , and  56  shown in  FIG. 3  is similar to the structure of the power switches  46  and  48 .  
         [0062]     Each drive is enabled by providing a logic 1 at the Q output of the buffered flip-flop associated with that drive, and disabled by providing a logic 0 at the output of the same buffered flip-flop. The buffered flip-flops are identified as buffered flip-flop  0   28 , buffered flip-flop  1   30 , . . ., and buffered flip-flop n- 1   32 . Each buffered flip-flop illustrated in  FIG. 3  may be regarded as having the structure formed by the first NAND gate  17 , the second NAND gate  19 , the third NAND gate  21 , and the fourth NAND gate  23  shown in  FIG. 2 , in combination with the resistors  13 ,  15 ,  77 ,  81 ,  79 , and  83  shown in the same figure.  
         [0063]     The buffered flip-flop  28  shown in  FIG. 3  will provide a logic 1 signal at its Q output terminal  84  when a logic 0 signal is applied at its input terminal  80  labeled S and will retain the logic 1 signal at its Q output terminal  84  thereafter until a logic 0 signal is applied at its input terminal  82  labeled R. The signal at the Q output  84  of the buffered flip-flop  28  then changes to a logic 0 signal. After the logic 0 signal appears at the Q output terminal  84  of the buffered flip-flop  28 , the buffered flip-flop  28  will retain the logic 0 signal at its Q output terminal  84  until a logic 0 signal is applied at its input terminal  80  labeled S. The other buffered flip-flops  30 , . . . ,  32  shown in  FIG. 3  operate in the same manner as the first buffered flip-flop  28 .  
         [0064]     A logic 0 signal is applied to buffered flip-flop  0   28  at the input terminal  80  labeled S via a diode  34  during the time interval in which the voltage on the charging capacitor  4  is low if the rotor  6  of the selector switch  14  is in contact with the first stator terminal  8  of the selector switch  14  during that interval. When a logic 0 signal is applied to buffered flip-flop  0   28  at the input terminal  80  labeled S, that logic 0 signal is applied also to buffered flip-flop  1   30 , . . . , and buffered flip-flop n- 1   32  at their input terminals  88 , . . . ,  92  labeled R, via isolating diodes  40 , . . . ,  26 . The isolating diodes  36 ,  22 ,  40 ,  24 , . . . ,  26 , and  44  are required to prevent logic 0 signals applied to the R input of one buffered flip-flop from affecting the R input of another buffered flip-flop. The diodes  34 ,  38 , . . . , and  42  are included to assure that the logic 0 signals presented to the flip-flops  28 ,  30 , . . . , and  32  at their input terminals  80 ,  86 , . . . , and  90  labeled S have the same voltage level as the logic signals presented to the same flip-flops at their inputs  82 ,  88 , . . . , and  92  labeled R.  
         [0065]     Similarly, a logic 0 signal can be applied via a diode  38  to buffered flip-flop  1   30  at its input terminal  86  labeled S and via isolating diodes  36 , . . . , and  44  to all of the other buffered flip-flops at their input terminals  82 , . . . , and  92  labeled R, if the rotor  6  of the selector switch  14  is in contact with terminal  10  of the selector switch  14 . In the same way, a logic 0 signal can be applied via a diode  42  to buffered flip-flop n- 1   32  at its input terminal  90  labeled S and via isolating diodes  22 ,  24 , . . . to all of the other buffered flip-flops  28 ,  30 , . . . at their input terminals  82 ,  88 , . . . labeled R, if the rotor  6  of the selector switch  14  is in contact with the stator terminal  12  of the selector switch  14 .  
         [0066]     Thus, by putting the rotor  6  of the selector switch  14  in contact with the appropriate stator terminal  8 ,  10 , . . . , or  12  before power is applied to the switching circuit, it is possible to enable any one of the drives  58 ,  60 , . . . ,  62  and disable all of the remaining drives for as long as power is present.  
         [0067]     Because the capacitor  4  charges through a resistor  2 , the voltage on the capacitor  4 , and hence the logic signal applied to the rotor  6  of the selector switch  14 , remains low for only a brief time after power is applied to the switching circuit; therefore, the selection of the disk drive to be enabled cannot be changed until after power has been removed from the switching circuit.  
         [0068]     In another version of this embodiment of the invention, provision is made also for the selection of any desired distinct combination of n disk drives. This can be accomplished, for example, by modifying the system illustrated in  FIG. 3  as follows. For each desired distinct combination of disk drives in  FIG. 3 , an additional stator terminal is added to the selector switch  14 ; then the cathodes of n additional diodes are connected to that stator terminal, and a connection is made from the anode of each of those diodes to the input terminal labeled S of the buffered flip-flop associated with a distinct drive in the combination to be selected and to the R input of the buffered flip-flop associated with a distinct one of all other drives. If there are to be m drives in the combination to be selected, there will be connections through diodes to the additional stator terminal of the selector switch  14  from the input terminals labeled S of m buffered flip-flops and connections through diodes to the same stator terminal of the selector switch  14  from the input terminals labeled R of n-m buffered flip-flops.  
         [0069]     If in a given digital system there can be only one master in operation at any given time, the master/slave jumpers on the various disk drives can be incorporated in the switching system so that, for any given position of the rotor  6  of the switch  14  there is only one master and the other enabled drives are configured as slaves. Such a requirement exists, for example, in IBM personal computers and IBM-compatible personal computers, where each disk drive controller cable can support only one master disk drive and a second disk drive connected to the same cable must be configured as a slave. In order for this configuration to be usable, the computer must allow operation with multiple operating systems present concurrently, however. Alternatively, the disks can be configured for cable select operation where permitted.  
         [0070]     In some embodiments of this invention, a first group of mass storage units is enabled at all times the system is in operation; and one or more of the remaining mass storage units in the system are identified to be enabled and the remainder disabled as described above.  
         [0071]     Thus, it is seen that in some preferred embodiments this invention comprises a system for a) selecting one or more of a multiplicity of mass storage units associated with a digital computer at a time when none of that group of mass storage units is operational or enabled, and b) preventing a change in selection after that one or more mass storage units have been enabled or made operational and the remainder have been disabled. The selection may be made by hardware, firmware, or software. The selection may be made by use of an independent switch, and it may be made by a modified power switch or by a modification of the computer shutdown/restart menu, for example.  
         [0072]     In other embodiments, after one mass storage unit has been selected by a hardware system and enabled, one or more other mass storage units may be enabled in addition, by hardware, firmware, or software, while at least one additional mass storage unit is disabled. This can be done, for example, by modifying the computer&#39;s BIOS and/or its configuration file, or by adding a second switch, to be actuated at some time after the first mass storage unit has been enabled.  
         [0073]     In still other embodiments of this invention, a digital computer with multiple mass storage units is booted in the conventional way; then, at a later time, one or more of the mass storage units are disabled in an orderly manner so as to prevent loss of data and/or damage to software, while the remaining mass storage units remain enabled, with rebooting if necessary on one of the mass storage units not disabled. In this way it is possible, for example, to provide protection against hackers from all other users of a network and achieve total privacy of all files on the mass storage units that were disabled. As a result, the disabled mass storage units are protected from viruses, worms, and all other forms of harmful intrusion transmitted over the network while still allowing uninhibited use of the network on the mass storage units that remain enabled.  
         [0074]     The enabling of one group of mass storage units and the disabling of others may be accomplished by hardware, firmware, or software. This can be done, for example, by modifying one or more signals in the control cables connected to one or more of the mass storage units, by modifying the computer&#39;s BIOS and/or its configuration file, by putting one or more of the mass storage units in the sleep mode or the standby mode, or by adding a second switch, to be actuated at some time after the first mass storage unit has been enabled.  
         [0075]     In still other embodiments of this invention, a first multiplicity of mass storage units may be enabled without switching, and a second multiplicity of mass storage units may be enabled by switching, as described previously.  
         [0076]     An example of an assembly without a mass memory unit, which is nevertheless encompassed by this invention, is illustrated in  FIG. 4 . Control apparatus  100  serves to interface a multiplicity of mass storage units such as drive  0   58  in  FIG. 3 , drive  1   60  in  FIG. 3 , . . . , and drive n- 1   62  in  FIG. 3  to the power supply in a digital computer. The control apparatus  100 , which comprises the enabling apparatus  103  shown in  FIG. 1  and may comprise also distinct selection apparatus  101  shown in  FIG. 1 , is connected to the +5V supply and the +12V supply in a digital computer by connecting apparatus  104 , which may comprise a connector to simplify connection and removal. For example, the connecting apparatus  104  may provide +5V to power switch  46  in  FIG. 3  on conductor  112 , to power switch  50  in  FIG. 3  on conductor  114 , . . . , and to power switch  54  in  FIG. 3  on conductor  116 ; and the same connecting apparatus  104  may provide + 12  V to power switch  48  in  FIG. 3  on conductor  122 , to power switch  52  in  FIG. 3  on conductor  124 , . . . , and to power switch  56  in  FIG. 3  on conductor  126 . Similarly, the control apparatus  100  is connected to the +5V supply line  128  and the +12V supply line  134  on drive  0   58  in  FIG. 3  by connecting apparatus  106 , which may contain a connector to facilitate connection to drive  0   58 , to the +5V supply line  130  and the +12V supply line  136  on drive  1   60  in  FIG. 3  by connecting apparatus  108 , which may contain a connector to facilitate connection to drive  1   60 , . . . , and to the +5V supply line  132  and the +12V supply line  138  on drive n- 1   62  in  FIG. 3  by connecting apparatus  110 , which may contain a connector to facilitate connection to drive n- 1   62 . The ground connections to drive  0   58 , drive  1   60 , . . . , and drive n- 1   62  may extend directly from the power supply connecting apparatus  104  to the disk drive connecting apparatus  106  for drive  0   58 , the connecting apparatus  108  for drive  1   60 , . . . , and the connecting apparatus  110  for drive n- 1   62 ; or the ground connections may pass through the control apparatus  100 .  
         [0077]     As has been mentioned above, one highly desirable aspect of this invention is that it provides privacy from other users of a common digital computer. Such privacy may be obtainable by cooperation among users, but it can be assured, if desired, by provision of a locking device, which may be electronic or mechanical or a combination of the two. One example of a lock appropriate for this purpose is shown in  FIG. 5 .  
         [0078]     Single-pole, single-throw switches  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 , and  226  are configured as an input keypad that is provided with a mechanical lock to prevent changes in the state of the keys by those who do not possess a key. If the switches are regarded as input keys in a natural binary representation, with the uppermost key  212  having the most significance and the lowermost key  226  having the least significance, then the electrical signals appearing at the inputs to the 8-input NAND gate  202  will form a natural binary code corresponding to the natural binary code entered via the switches  212 ,  214 , . . . ,  226 . The output of the NAND gate  202 , which may be a type 7430 integrated circuit, for example, is inverted by a two-input open-collector NAND gate  200  operating as an inverter, with the result that the output voltage of the two-input open-collector NAND gate  200  is high if and only if the eight inputs to the NAND gate  202  are all high. The two-input open-collector NAND gate  200  may be a part of a type 7403 quad two-input open-collector NAND gate, for example.  
         [0079]     Because the outputs of input keys  212 ,  216 ,  218 , and  224  are inverted, by inverters  204 ,  206 ,  208 , and  210 , respectively, the eight inputs to the 8-input NAND gate  202  will all be high if and only if the input code entered via the keypad is 01001101. Each of the inverters  204 ,  206 ,  208 , and  210  may be a part of a type 7404 hex inverter, for example.  
         [0080]     The output terminal of the two-input open-collector NAND gate  200  may be connected via a conductor  230  to the conductor  99  at the output of NAND gate  21  in  FIG. 2 , for example. Then the output of the NAND gate  21  will be held low by NAND gate  200 , regardless of whether drive A  73  in  FIG. 2  is selected by the selector switch  1 , unless the code entered on the keypad comprising keys  212 ,  214 , . . . ,  226  is 01001101. Thus, all users of the common digital computer except for those who know the code and have a key that provides access to the keypad for data entry are prevented from activating drive A  73 .  
         [0081]     Circuitry similar to that shown in  FIG. 5 , with distinct access codes, can be connected to one or more of the input lines, such as conductor  84  in  FIG. 3 , for example, to the enabling apparatus in a system such as that illustrated in  FIG. 3 . Thus, access to all of the mass storage units, such as drive  0   58 , drive  1   60 , . . . , and drive n- 1   62  or any desired subset thereof can be limited.  
         [0082]     Optionally, a different kind of electronic lock, similar to that used to limit access to a home, for example, may be used instead of the lock shown in  FIG. 5 . In some such locks, the input keys  212 ,  214 , . . . ,  226  are replaced with momentary contact keys, and a form of memory is added to retain the last code or combination entered. As a result, there is no need for a mechanical lock. Such an electronic lock can also be connected to the conductor  99  at the output of NAND gate  21  in  FIG. 2 , for example, to achieve the same result as the lock illustrated in  FIG. 5 .  
         [0083]     Although the invention disclosed herein has been described with reference to specific embodiments, various modifications and improvements will occur to those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular forms illustrated, nor to particular devices known at present, but includes all arrangements of apparatus that do not depart from the spirit and scope of the appended claims and specific devices now known or to be developed in the future.