Abstract:
A data storage device includes one or more non-volatile, blockwise erasable data storage media and a mechanism for sanitizing the media in response to a single external stimulus or in response to a predetermined physical or logical condition. Optionally, only part of the media is sanitized, at a granularity finer than the blocks of the medium. Setting a flag in an auxiliary nonvolatile memory enables an interrupted sanitize to be detected and restarted. Optionally, a “death certificate” verifying the sanitizing is issued. Preferably, the media are configured in a manner that allows atomic operations of the sanitizing to be effected in parallel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of prior application Ser. No. 11/171,382, filed on Jul. 1, 2005, which is a divisional of U.S. Pat. No. 7,003,621, issued on Feb. 21, 2006, which is a continuation-in-part of U.S. Provisional Patent Application No. 60/457,021, filed on Mar. 25, 2003. The disclosure of each above-identified application is incorporated herein by reference. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to nonvolatile storage devices and, more particularly, to methods for sanitizing a flash-based data storage device and to a flash-based data storage device particularly adapted to the implementation of these methods. For as long as data has been stored digitally, there has been a need to erase classified data, from the medium in which they are stored, in a manner that renders the data unrecoverable. Such an erasure is called “sanitizing” the medium. 
         [0003]    The most common nonvolatile data storage devices use magnetic data storage media, in which data bits are stored as magnetized regions of a thin ferromagnetic layer. It is difficult to sanitize such a medium. The usual method of sanitizing such a medium is to write over the data many times with different data patterns. This method requires a long time (minutes to hours) to perform, and cannot be guaranteed to render the old data unrecoverable. A sufficiently well-equipped laboratory can reconstruct data that were overwritten many times. Alternatively, the medium can be sanitized by degaussing it. Degaussing devices are cumbersome, power-hungry devices that are external to the system whose data storage medium is to be sanitized. Degaussing is considered safer than overwriting multiple times but is still not foolproof. The only foolproof way to sanitize a magnetic storage medium is to destroy it physically, which obviously renders the medium no longer useable to store new data. 
         [0004]    More recently, a form of EEPROM (electronically erasable programmable read-only memory) non-volatile memory called “flash” memory has come into widespread use.  FIG. 1  is a high level schematic block diagram of a generic flash-based data storage device  10  for storing data in one or more flash media  12 , for example NAND flash media. The operation of device  10  is controlled by a microprocessor-based controller  14  with the help of a random access memory (RAM)  16  and an auxiliary non-volatile memory  18 . Flash device  10  is used by a host device  24  to store data in flash media  12 . Flash device  10  and host device  24  communicate via respective communication ports  20  and  26  and a communication link  23 . Typically, for backwards compatibility with host devices  24  whose operating systems expect magnetic storage devices, flash device  10  emulates a block memory device, using firmware stored in auxiliary non-volatile memory  18  that implements the methods taught by Ban in U.S. Pat. No. 5,404,485 and U.S. Pat. No. 5,937,425, both of which patents are incorporated by reference for all purposes as if fully set forth herein. 
         [0005]    The “atomic” operations that controller  14  performs on flash media  12  include read operations, write operations and erase operations. One important property of flash media  12  that is relevant to the present invention is that the granularity of the erase operations is larger than the granularity of read and write operations. For example, a NAND flash medium typically is read and written in units called “pages”, each of which typically includes between 512 bytes and 2048 bytes, and typically is erased in units called “blocks”, each of which typically includes between 16 and 64 pages. Various US government agencies (primarily military) have defined standards for sanitizing flash media  12 . According to DoD 5220.22-M National Industrial Security Program Operating Manual (NISPOM), every byte in flash media  12  is overwritten with the same character, and then flash media  12  are erased. According to National Security Agency (NSA) Manual 130-2, US Air Force System Security Instructions (AFSSI) 5020 and US Navy Staff Office Publication (NAVSO) 5239, “Information System Security Program Guidelines” (INFOSEC), flash media  12  are first erased and then are overwritten with random data. According to US Army Regulation 380-19, Information System Security, flash media  12  are first erased and then overwritten twice. In the first overwrite, flash media  12  are overwritten with random data. In the second overwrite, every byte in flash media  12  is overwritten with the same character. Finally, flash media  12  are erased a second time. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention defines several improvements to the prior art methods of sanitizing flash media and to the flash devices being sanitized. Although the description herein is directed towards the sanitation of flash media, the scope of the present invention extends to all non-volatile data storage media to which the principles of the present invention are applicable. 
         [0007]    According to the present invention there is provided a method of cleaning a medium wherein data are stored, the medium including a plurality of blocks and that is only block-wise erasable, each block being bounded by a respective first block boundary and a respective second block boundary, the method including the steps of: (a) selecting a portion of the medium to sanitize, the portion being bounded by a first portion boundary and a second portion boundary, at least one of the portion boundaries being within one of the blocks; (b) for each of the portion boundaries that is within one of the blocks, copying the data, that is stored in the one block outside of the portion, to a second block; and (c) sanitizing every block spanned by the portion. 
         [0008]    According to the present invention there is provided a data storage device including: (a) a data storage medium; and (b) a mechanism for sanitizing the data storage medium in response to a single external stimulus. 
         [0009]    According to the present invention there is provided a method of cleaning a data storage medium, including the steps of: (a) setting a flag that indicates that the data storage medium is to be sanitized; and (b) subsequent to the setting, beginning a first sanitizing of the data storage medium. 
         [0010]    According to the present invention there is provided a data storage device including: (a) a data storage medium; and (b) a controller for sanitizing the data storage medium upon detection of a predetermined condition. 
         [0011]    According to the present invention there is provided a method of cleaning a data storage medium, including the steps of: (a) sanitizing the data storage medium; and (b) subsequent to the sanitizing, setting a medium-is-sanitized flag. 
         [0012]    According to the present invention there is provided a data storage device including: (a) at least one plurality of data storage media; and (b) a controller for, for each at least one plurality of the data storage media: (i) writing data, substantially simultaneously, to at least a portion of each of the data storage media of the each plurality, and (ii) erasing, substantially simultaneously, at least a portion of each of the data storage media of the each plurality. 
         [0013]    According to the present invention there is provided a method of cleaning a data storage device that includes at least one plurality of data storage media, including the steps of: (a) selecting a sanitize procedure, the sanitize procedure including at least one atomic operation; and (b) for each at least one plurality of data storage media: applying the selected sanitize procedure to the data storage media of the each plurality, with each at least one atomic operation being applied substantially simultaneously to the data storage media of the each plurality. 
         [0014]    The first improvement of the present invention is directed towards selectively sanitizing only a portion of a flash medium, or more generally, only a portion of a data storage medium that is erased in blocks and that is read and written in units that are smaller than the blocks. Specifically, this method is directed towards sanitizing a portion of the medium, one or both of whose boundaries do not coincide with block boundaries. For each portion boundary that falls between the two boundaries of one of the blocks, the data stored in that block that fall outside the portion to be sanitized first are copied to a second block. Only then are the block or blocks, that are spanned by the portion of the medium to be sanitized, actually sanitized. For this to work, the second block must be outside (i.e., not spanned by) the portion to be sanitized. 
         [0015]    Preferably, the second block is itself sanitized before the data from just beyond the portion to be sanitized are copied to the second block. 
         [0016]    Preferably, at least one free block that is outside the portion to be sanitized also is sanitized. 
         [0017]    The second improvement of the present invention is a data storage device that includes a (preferably non-volatile) data storage medium and a mechanism for sanitizing the data storage medium in response to a single external stimulus, as opposed to, for example, a sequence of several commands from host device  24  that instruct controller  14  to implement one of the sanitization standards discussed above. Although these standards have been in use at least since 1990, such a data storage device has not been implemented heretofore. 
         [0018]    According to one aspect of the second improvement, the mechanism includes an interface to a host system, and the external stimulus is a single “sanitize” command from the host system. 
         [0019]    According to another aspect of the second improvement, the mechanism includes an interrupt handler, and the external stimulus is a hardware interrupt. To this end, the data storage device also includes an interrupt initiator for providing the hardware interrupt. Preferably, the interrupt initiator includes a wireless transmitter for transmitting the hardware interrupt, and the interrupt handler includes a wireless receiver for receiving the transmitted hardware interrupt. 
         [0020]    The third improvement of the present invention is a method of sanitizing a data storage medium that can be restarted after being interrupted, for example by a power failure. Before starting a first sanitizing of the data storage medium, a flag is set that indicates that the data storage medium is to be sanitized. Upon completion of the first sanitizing, the flag is cleared. 
         [0021]    Preferably, before the beginning of the first sanitizing, at least one sanitizing parameter is stored. Upon completion of the first sanitizing, the at least one parameter is erased. 
         [0022]    When the data storage medium is powered up, the flag is checked. If the flag is set, indicating that the first sanitizing was interrupted, a second sanitizing of the data storage medium is begun. Upon completion of the second sanitizing, the flag is cleared. Preferably, if the at least one sanitizing parameter was stored before beginning the first sanitizing, then upon completion of the second sanitizing, the at least one sanitizing parameter is erased. 
         [0023]    The fourth improvement of the present invention is a data storage device that supports conditional sanitization. The device includes a (preferably non-volatile) data storage medium and a controller for sanitizing the data storage medium upon detection of a predetermined condition. 
         [0024]    Preferably, the condition is a physical condition, such as an interruption of power or an improper shutdown, or else a logical condition. Preferably, the logical condition is an indication that an unauthorized access of the data storage medium has been attempted. One example of such a logical condition is more than a predetermined number of accesses (e.g., reads or writes) to a preselected datum, for example a FAT table entry, that is stored in the data storage medium. Another example of such a logical condition is more than a predetermined number of accesses (e.g., reads, writes or erases) to a preselected portion of the data storage medium. 
         [0025]    The fifth improvement of the present invention is a method of sanitizing a data storage medium that supports the provision of a “death certificate” for the sanitized medium. A “medium is sanitized” flag is set after the data storage medium is sanitized. Once the flag has been set, it can be verified that the data storage medium has been sanitized by checking that the flag is indeed set. Preferably, the verifying also includes checking at least a portion of the data storage medium for a data pattern stored therein (including “no data” if the last step of the sanitizing process was an erase) that indicates that the data storage medium has been sanitized. Most preferably, the entire data storage medium is checked for a data pattern stored therein that indicates that the data storage medium has been sanitized. 
         [0026]    Preferably, if the verifying determines that the data storage medium has in fact been sanitized, a death certificate for the data storage medium is issued. Most preferably, the death certificate is based on a verification seed and on a serial number of the data storage device that includes the data storage medium. 
         [0027]    The sixth improvement of the present invention is a data storage device that supports parallel sanitizing, and a method of sanitizing the device. 
         [0028]    The device includes at least one plurality, and preferably more than one plurality, of data storage media, and a controller for writing data, substantially simultaneously, to at least a portion of each data storage medium of each plurality, and for erasing, substantially simultaneously, at least a portion of each data storage medium of each plurality. Note that all of the sanitization standards discussed above include both writes and erases. Preferably, the device also includes, for each plurality of data storage media, at least one respective bus that operationally connects the data storage media of the plurality to the controller. 
         [0029]    Preferably, the data storage media are non-volatile. Most preferably, the data storage media are NAND flash chips. 
         [0030]    Preferably, the data storage media are page-wise writable. Preferably, the portion of each data storage medium to which data are written during a substantially simultaneous write is a single page of the data storage medium. Alternatively, the portion of each data storage medium to which data are written during a substantially simultaneous write is a plurality of pages of the data storage medium. Another alternative is to write the data to all of each data storage medium of the plurality, i.e., to every page of each data storage medium of the plurality, not just to portions of the data storage media, during a substantially simultaneous write. 
         [0031]    Preferably, the data storage media are block-wise erasable. Preferably, the portion of each data storage medium that is erased during a substantially simultaneous erase is a single block of the data storage medium. Alternatively, the portion of each data storage medium that is erased during a substantial simultaneous erase is a plurality of blocks of the data storage medium. Another alternative is to erase all of each data storage medium of the plurality, i.e., to erase every block of each data storage medium, not just portions of the data storage media, during a substantially simultaneous erase. 
         [0032]    The method of the sixth improvement has two steps. In the first step, a sanitize procedure for the data storage device is selected. This procedure includes at least one atomic operation. Typically, as in the sanitize standards discussed above, the atomic operations are writes and erases, although the procedure could include reads, for example if the procedure is directed at only a portion of each data storage medium. In the second step, the procedure is applied to the data storage media, with each atomic operation being applied substantially simultaneously to the data storage media of each plurality of data storage media. 
         [0033]    The substantially simultaneous atomic operation may be a substantially simultaneous write of data to a single page of each data storage medium of a plurality of data storage media, a substantially simultaneous write of data to a plurality of pages of each data storage medium of a plurality of data storage media, or a substantially simultaneous write of data to all (i.e., to every page) of each data storage medium of a plurality of data storage media. The substantially simultaneous atomic operation may be a substantially simultaneous erase of a single block of each data storage medium of a plurality of data storage media, a substantially simultaneous erase of a plurality of blocks of each data storage medium of a plurality of data storage media, or a substantially simultaneous erase of all (i.e., of every block) of each data storage medium of a plurality of data storage media. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0035]      FIG. 1  is a high level schematic block diagram of a prior art flash-based data storage device coupled to a host device; 
           [0036]      FIG. 2  is a high level schematic block diagram of a flash-based data storage device of the present invention coupled to the host device of  FIG. 1 ; 
           [0037]      FIG. 3  shows the internal structure of the flash array of the data storage device of  FIG. 2 ; 
           [0038]      FIG. 4  shows the internal partition into blocks and pages of a NAND flash chip of the flash array of  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    The present invention is of improved methods of sanitizing data storage media, and of data storage devices that support these methods. Specifically, the present invention can be used to sanitize flash-based data storage media such as NAND flash chips. The principles and operation of data storage media sanitization according to the present invention may be better understood with reference to the drawings and the accompanying description. 
         [0040]    Referring again to the drawings,  FIG. 2  is a high-level schematic block diagram of a flash-based data storage device  30  of the present invention, coupled to host device  24  of  FIG. 1 . Most of the high level components of device  30  are the same as in prior art device  10 , although the controller and the auxiliary non-volatile memory of device  30  are given different reference numerals ( 34  and  38  respectively) to indicate that these components are different functionally, if not structurally, from controller  14  and auxiliary non-volatile memory  18  of device  10 . Controller  34  and auxiliary non-volatile memory  38  have all the functionality of prior art controller  14  and prior art auxiliary non-volatile memory  18 , and also functionality of the present invention, as discussed below. 
         [0041]    In place of flash media  12 , device  30  is shown as including a flash array  32  that is illustrated in more detail in  FIG. 3 . Flash array  32  includes several subarrays  40 A through  40 N of NAND flash chips  42 . Each subarray  40  includes the same number (between 2 and 64) of NAND flash chips  42 . In the illustrated example, each subarray  40  includes four NAND flash chips  42 . NAND flash chips  42  of each subarray  40  communicate with controller  34  via a corresponding set  44  of buses, either four 32-bit buses or two 64-bit buses per set. 
         [0042]    For reference,  FIG. 4  shows the structure of a NAND flash chip  42 . NAND flash chip  42  includes between 1024 and 8192 blocks  46 . Every NAND flash chip  42  of a particular subarray  40  includes the same number of blocks  46 . Every block  46  includes the same number of pages  48 , either 16 pages  48  per block  46 , 32 pages  48  per block  46  or 64 pages  48  per block  46 . Every page  48  includes the same number of bytes, which number could be any multiple of 512 between 512 and 2048. As described above, the erasable units of NAND flash chip  42  are blocks  46  and the readable and writable units of NAND flash chip  42  are pages  48 . 
         [0043]    Typical NAND flash chips  42  support one or both of two kinds of erase commands. A block erase command erases a designated block  46 . A multi-block erase command erases a designated group of blocks  46 , typically four blocks  46 . Similarly, typical NAND flash chips  42  support one or both of two kinds of write commands. A page write command writes one page worth of data from RAM  16  (used as a buffer) to a designated page of a designated block  46 . A multi-page write command writes several pages, typically four pages, worth of data from RAM  16  to several designated pages of a designated block  46 . 
         [0044]    While a NAND flash chip is executing an erase or write command, the NAND flash chip sets its status to “busy”. Upon completing the execution of the command, the NAND flash chip sets its status to “ready”. According to the prior art, when prior art flash media  12  are NAND flash chips, after prior art controller  14  issues a write or erase command to any particular NAND flash chip, prior art controller  14  waits for that NAND flash chip&#39;s status to change from “busy” to “ready” before issuing the next command of the same type (erase or write). The architecture of flash array  32 , as illustrated in  FIG. 3 , allows enhanced parallelism in sanitizing flash array  32 . Specifically, within each subarray  40 , controller  34  issues, via buses  44 , successive erase or write commands to all NAND flash chips  42  of that subarray  40 , without waiting for any NAND flash chip  42  to transit from “busy” status to “ready” status before issuing the erase or write command to the next NAND flash chip  42 . In this manner, all NAND flash chips  42  of a subarray  40  are erased, or written to, substantially simultaneously. As a result, with N NAND flash chips  42  per subarray  40 , sanitizing flash array  32  is almost N times faster than sanitizing comparable prior art flash media  12 . 
         [0045]    For example, sanitizing flash array  32  according to the NISPOM standard includes two phases, a write phase and an erase phase. For definiteness, this example uses page write and block erase commands. 
         [0046]    In the write phase, one page&#39;s worth of the overwrite character is loaded into a one-page-long buffer in RAM  16 . The remainder of the phase consists of four nested loops: an outer loop, an intermediate loop within the outer loop, and two inner loops within the intermediate loop. The outer loop is over page number. The intermediate loop is over subarrays  40 . The first inner loop is over NAND flash chips  42  of the current subarray  40 : in each cycle of the loop, controller  34  issues a page write command to copy the buffer in RAM  16  to the current page  48  of the current NAND flash chip  42 , without having waited for the immediately preceding NAND flash chip  42  to enter “ready” status. The second inner loop also is over NAND flash chips  42  of the current subarray  40 : in each cycle of the loop, controller  34  inspects the status of the current NAND flash chip  42 . The second inner loop is repeated until all NAND flash chips  42  of the current subarray  40  are in “ready” status. 
         [0047]    The erase phase also has four nested loops: an outer loop, an intermediate loop within the outer loop, and two inner loops within the intermediate loop. The outer loop is over block number. The intermediate loop is over subarrays  40 . The first inner loop is over NAND flash chips  42  of the current subarray  40 : in each cycle of the loop, controller  34  issues a block erase command to erase the current block  46  of the current NAND flash chip  42 , without having waited for the immediately preceding NAND flash chip  42  to enter “ready” status. The second inner loop also is over NAND flash chips  42  of the current subarray  40 : in each cycle of the loop, controller  34  inspects the status of the current NAND flash chip  42 . The second inner loop is repeated until all NAND flash chips  42  of the current subarray  40  are in “ready” status. 
         [0048]    Sanitizing flash array  32  with multi-page write commands and multi-block erase commands is similar, with the outer loops being over groups of pages  48  and blocks  46  instead of over individual pages  48  and blocks  46 . 
         [0049]    NOR flash chips support, in addition to block erase page write commands, chip erase commands that erase entire chips, not just individual blocks/pages. It is expected that NAND flash chips soon will be available that support both such chip erase commands and also chip write commands that write entire chips; and that NOR flash chips also soon will be available that support both chip erase commands and chip write commands. When such NAND flash chips are available, sanitizing flash array  32  still will be as described above, except that there will be no outer loops over (groups of) pages or over (groups of) blocks. 
         [0050]    Returning to  FIG. 2 , device  30  also includes an interrupt handler  50 , which is shown separate from controller  34  but which alternatively could be integrated in controller  34 . A user of device  30  initiates sanitizing of flash array  32  by using an interrupt initiator  52  to signal interrupt handler  50 . This signal is a hardware interrupt that causes controller  34  to immediately stop whatever activity controller  34  is currently engaged in and to start sanitizing flash array  32 . In one preferred embodiment of device  30 , interrupt initiator  52  is an electrical switch that is operated manually by the user and that is connected to interrupt handler  50  by wires. In another preferred embodiment of device  30 , interrupt initiator  52  is an electrical system that automatically initiates sanitizing of flash array  32  in an emergency. In yet another preferred embodiment of device  30 , which is the embodiment actually illustrated in  FIG. 2 , interrupt initiator  52  is a manually or automatically operated transmitter of wireless electromagnetic signals and interrupt handler  50  is a receiver of those signals. Interrupt initiator  52  transmits an appropriate electromagnetic signal  54  to interrupt handler  50  to initiate sanitizing of flash array  32 . Suitable communication standards for interrupt initiator  52  and interrupt handler  50  in this preferred embodiment include Bluetooth for radio frequency signals and IrDA for infrared signals. 
         [0051]    More generally, according to the present invention, sanitizing of flash array  32  is initiated by a single external stimulus. The hardware interrupt initiated by interrupt initiator  52  is one example of such an external stimulus. Another example of such an external stimulus is a software interrupt in the form of a “sanitize” command received by controller  34  from host  24 . This is in contrast to the prior art of  FIG. 1 , in which host  24  must send to device  10  the explicit sequence of write and erase commands that sanitize flash media  12 . Although the various standards described above for sanitizing flash media  12  have been in use since 1990, the data storage device of the present invention is the first such data storage device whose data storage medium can be sanitized in response to a single external stimulus. 
         [0052]    To enable sanitizing of flash array  32  in response to a hardware interrupt, parameters that describe a default sanitize method (either one of the standard methods described above or a user-defined method) are stored in non-volatile memory  38 . When interrupt handler  50  receives the hardware interrupt signal, controller  34  reads these parameters from non-volatile memory  38  and proceeds accordingly. In the case of a sanitize initiated by a software interrupt, the sanitize command from host  24  optionally is optionally accompanied by sanitize parameters that override the default sanitize parameters that are stored in non-volatile memory  38 . 
         [0053]    Controller  34  also sanitizes flash array  32  upon detection of a predetermined condition. This condition may be either a physical condition or a logical condition. One typical physical condition is an interruption of power that is detected by a reset chip (not shown) in device  30 . Upon detection of the interruption of power, the reset chip initiates an interrupt via interrupt handler  50 . Controller  34  then sanitizes flash array  32  either upon the next power-up or, alternatively, immediately using a back-up power source (not shown). Another typical physical condition is an improper shutdown of device  30 . 
         [0054]    The logical condition typically is a condition that suggests an attempted unauthorized access of the data stored in flash array  32 . One example of such a logical condition is that a predetermined datum, such as a FAT table entry, has been accessed (read and/or written) more than a predetermined number of times. Another example of such a logical condition is that a predetermined portion, such as a particular page  48  or block  46 , of flash array  32  has been accessed (read, written or erased) more than a predetermined number of times. 
         [0055]    Optionally, a wireless interrupt initiator  52  and interrupt handler  50  are configured to enable a user, not just to initiate the sanitizing of flash array  32 , but to handle all aspects of the sanitizing of flash array  32 . For example, a suitably configured interrupt initiator  52  and interrupt handler  50  can be used to set the default sanitize parameters, to override the default sanitize parameters, or to interrogate the sanitize status (sanitize not started, sanitize in progress or sanitize completed) of device  30 . 
         [0056]    Another important aspect of the present invention is the ability to sanitize only a selected part of flash array  32 , at a granularity finer than the level of blocks  46 . This ability relies on the methodology for managing flash data storage media that is taught in U.S. Pat. No. 5,404,485 and U.S. Pat. No. 5,937,425. According to this prior art methodology, controller  34  maintains a table, either in RAM  16  or in non-volatile memory  18  or even (see U.S. Pat. No. 5,404,485) in flash array  32  itself, that maps logical blocks and logical pages addressed by host  24  into the physical blocks and physical pages in flash array  32  in which data actually are stored. For example, a page  48  of a NAND flash chip  42  can be written to only a small (typically 3 to 10) number of times before that page must be erased in order to be rewritten. Therefore, it often happens that in order to replace a page  48  of old data with new data, controller  34  copies all the data stored in the physical block  46  in which the target page  48  is located, except for the data in the target page  48 , to all but one of the pages  48  a so-called “free” block, i.e., a physical block  46  that has not been written to since the last time it was erased, and writes the new data to the remaining page  48  of the new block  46 . Meanwhile, the table that maps logical blocks and pages to physical blocks and pages is updated so that the logical blocks and pages that were associated with the old physical block  46  and its pages  48  now are associated with the new physical block  46  and its pages  48 . This all is totally transparent to host  24 . As far as host  24  is concerned, the new data were written to the same (logical) page as the old data. 
         [0057]    It now will be explained how this methodology is used to facilitate partial sanitizing at a finer granularity than the level of physical blocks  46 . For this purpose, the notation (b,p) is used to represent the p-th page  48  of the b-th block  46 , and the notation (b,) is used to represent the b-th block  46 . It is assumed that every block  46  has P pages  48 , indexed 0 through P−1. 
         [0058]    Suppose that it is desired to sanitize pages (b.sub.i,p.sub.i) through (b.sub.f,p.sub.f), where b.sub.i.ltoreq.b.sub.f. (The subscript “i” means “initial”. The subscript “f” means “final”.) If p.sub.i=0 and p.sub.f=P−1, then all that is necessary is to sanitize blocks (b.sub.i,) through (b.sub.f,) according to the standards described above, which include erasures of entire blocks  46 , because the boundaries of the portion of flash array  32  that is to be sanitized coincide with block boundaries: the initial boundary of the first page to be sanitized coincides with the initial boundary of the first block and the final boundary of the last page to be sanitized coincides with the final boundary of the last block. But if p.sub.i&gt;0, then the initial boundary of the first page to be sanitized falls between the two boundaries of the first block, and the data in pages (b.sub.i,0) through (b.sub.i,p.sub.i−1) must be preserved. Similarly, if p.sub.f&lt;P−1 then the final boundary of the last page to be sanitized falls between the boundaries of the last block, and the data in pages (b.sub.f,p.sub.f+1) through (b.sub.f,P−1) must be preserved. Therefore, if p.sub.i&gt;0, pages (b.sub.i,0) through (b.sub.i,p.sub.i−1) first are copied to a free block  46 . Similarly, if p.sub.f&lt;P−1, pages (b.sub.f,p.sub.f+1) through (b.sub.f,P−1) first are copied to a free block  46 . Only then are blocks (b.sub.i,) through (b.sub.f,), that span the targeted portion of flash array  32 , sanitized. Most preferably, the free block  46  to which pages (b.sub.i,0) through (b.sub.i,p.sub.i−1) are copied is itself sanitized before the pages are copied, and the free block  46  to which pages (b.sub.f,p.sub.f+1) through (b.sub.f,P−1) are copied is itself sanitized before the pages are copied. Also most preferably, after blocks (b.sub.i,) through (b.sub.f,) are sanitized, all the remaining free blocks also are sanitized, to make sure that any nominally free blocks that contain out-of-date or superceded classified data are sanitized. Finally, the table that maps logical blocks and pages to virtual blocks and pages is updated to reflect the new physical locations of the data formerly stored in physical pages (b.sub.i,0) through (b.sub.i,p.sub.i−1) and/or in physical pages (b.sub.f,p.sub.f+1) through (b.sub.f,P−1). 
         [0059]    Another important aspect of the present invention is the ability to complete a sanitizing that was interrupted by, for example, a power failure. To this end, before starting to sanitize flash array  32 , controller  34  sets, in non-volatile memory  38 , a “sanitize-on” flag that indicates that flash array  32  is to be sanitized. If the sanitize was initiated by a software interrupt accompanied by sanitize parameters that override the default sanitize parameters, controller  34  also stores these new sanitize parameters in non-volatile memory  38 , separately from the default sanitize parameters. 
         [0060]    Controller  34  then starts to sanitize flash array  32 . After flash array  32  has been sanitized, controller  34  clears the sanitize-on flag. If the default sanitize parameters were overridden, controller  34  also erases the new sanitize parameters. 
         [0061]    Whenever device  30  is powered up, controller  34  s the sanitize-on flag. If the sanitize-on flag is set, that indicates that a sanitize of flash array  32  has been interrupted. Controller  34  therefore starts to sanitize flash array  32 , in accordance with the relevant sanitize parameters stored in non-volatile array  38 . After flash array  32  has been sanitized, controller  34  clears the sanitize-on flag. If the default sanitize parameters were overridden, controller  34  also erases the new sanitize parameters. 
         [0062]    The above description applies to resumption of an interrupted sanitize of all of flash array  32 . An interrupted partial sanitize of flash array  32  also can be resumed, using techniques adapted from Lasser et al., U.S. Pat. No. 6,977,847, which is incorporated by reference for all purposes as if fully set forth herein. Note that some of these techniques require modification of NAND flash chips  42 . 
         [0063]    After flash array  32  has been sanitized, controller  34  also sets, in non-volatile memory  38 , a “medium-is-sanitized” flag that remains set until the next time that data are written to flash array  32 . The presence of this medium-is-sanitized flag allows the fact that flash array  32  has been sanitized to be verified: if the medium-is-sanitized flag is set, then flash array  32  has been sanitized, and if the medium-is-sanitized flag is not set, then flash array  32  has not been sanitized. 
         [0064]    Optionally, a verification level parameter is stored in non-volatile memory  38 . The values of this verification level parameter are indicative of one of three different verification levels: 
         [0065]    Level 1: check only the medium-is-sanitized flag, as described above. 
         [0066]    Level 2: as in level 1, but also check a predetermined portion of flash array  32 , for example the first page  48  of every block  46 , for the presence of the data pattern that would be expected therein if those pages  48  actually have been sanitized. For example, if flash array  32  was sanitized according to the standard of US Army Regulation 380-19, every byte of those pages  48  should contain the same character. 
         [0067]    Level 3: as in level 2, but check all of flash array  32  for the presence of the expected data pattern. 
         [0068]    Optionally, a sanitize-verification-seed parameter is used to compute a “death certificate” for device  30 . This parameter is either stored in non-volatile memory  38  or received from the external device (host  24  or a suitably configured wireless interrupt initiator  52 ) that requests the verification of the sanitizing of flash array  32 . If, as checked according to the verification level determined by the verification level parameter, flash array  32  indeed has been sanitized, then a “death certificate” is computed, from the sanitize-verification seed and from the serial number of device  30  (which also is stored in nonvolatile memory  38 ), using a secret algorithm that is pre-defined by the user. The death certificate then is transmitted to the external device that requested the verification. 
         [0069]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.