An auxiliary storage device providing independent backup and recovery functions, the auxiliary storage device comprising: a primary storage device which is connected to a computer unit through a communication control unit to store and provide data; and a backup storage device , wherein the backup storage device comprises an input/output unit for controlling the communication control unit by an input/output of a user, and a mode switch which is connected to the input/output unit and configured to allow the user to select a normal mode or a recovery mode; and the backup storage device is configured to receive communication between the computer unit and the primary storage device in real time through the communication unit to generate and record backup information, and to output a control signal for controlling the communication control unit , wherein the communication control unit comprises a processor to receive input signals from the user and from the mode selection of the mode switch and to control communication between the computer unit and the primary storage device and communication between the computer unit and the backup storage device , wherein, when the user sets the mode switch to the normal mode, the communication control unit operates so that the computer unit only accesses the primary storage device and so that the backup storage device also recognizes communication information including contents transmitted from the computer unit to the primary storage device ; and when the user sets the mode switch to the recovery mode and reboots the computer unit , the communication control unit operates so that access to the primary storage device of the computer unit is blocked and the backup storage device is connected to the computer unit . a primary storage device which is connected to a computer unit through a communication control unit to store and provide data; and a backup storage device , wherein the backup storage device comprises an input/output unit for controlling the communication control unit by an input/output of a user, and a mode switch which is connected to the input/output unit and configured to allow the user to select a normal mode or a recovery mode; and the backup storage device is configured to receive communication between the computer unit and the primary storage device in real time through the communication unit to generate and record backup information, and to output a control signal for controlling the communication control unit , wherein the communication control unit comprises a processor to receive input signals from the user and from the mode selection of the mode switch and to control communication between the computer unit and the primary storage device and communication between the computer unit and the backup storage device , wherein, when the user sets the mode switch to the normal mode, the communication control unit operates so that the computer unit only accesses the primary storage device and so that the backup storage device also recognizes communication information including contents transmitted from the computer unit to the primary storage device ; and when the user sets the mode switch to the recovery mode and reboots the computer unit , the communication control unit operates so that access to the primary storage device of the computer unit is blocked and the backup storage device is connected to the computer unit . <EOS>