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  CLASS:  SSDevice
 
  The SSDevice class handles the low-level details for reading and writing
  sectors to and from a disk device.  It is an abstract base class from which
  true implementations should be derived.
 
  Public Methods:
 
  DriveDevice():  Constructor.  Takes device file pathname as argument.
  ~DriveDevice():  Destructor.  Closes device.
  numSectors():  Returns the number of sectors on the device.
  sectorSize():  Returns the size, in bytes, of a sector on the device.
  read():  Read bytes into argument ByteArray from the given ExtentList.
  write():  Write bytes from argument ByteArray into the given ExtentList.
  writeWOE():  As write(), but does a write-without-erase.
  verifyMode():  Setter: puts DriveDevice in write-with-verify mode.
  verifyMode():  Getter: returns write-with-verify mode flag.
  erase():  Erases the sectors specified by the given PhysExtent.
  checkValidity():  Returns whether the DriveDevice is in a valid state.
  error():  Returns the error state of the DriveDevice object.
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