The DoubleSpace API (INT 2fH 4a11H and related IOCTL fns) may return the
following error codes:

INT 2fH Error Codes

0000H I2F_NO_ERR              fn was successful
0100H I2F_ERR_BAD_FN          invalid fn number in BX
0101H I2F_ERR_BAD_DRV         invalid drive number specified
0102H I2F_ERR_NOT_COMPR       specified drive number is not compressed
0103H I2F_ERR_ALREADY_SWAPPED host drive already swapped
0104H I2F_ERR_NOT_SWAPPED     drive is not swapped (I assume that this is
returned by an undocumented fn)

IOCTL and INT 2fH 4a11H 0005H (mount volume) Error Codes

4f4bH OK_NO_ERROR             The IOCTL calls return 'OK' in the bResult
field of the IOCTL packet if no error.

01H LETTER_BOUNDARY_ERROR   Drive letter not available for DoubleSpace
(use a drive letter between FirstDrive and
LastDrive; see INT 2fH 4a11H 0000H)
02H UNIT_USED_ERROR         Drive letter already in use (use an unused
drive letter)
03H NO_FREE_SLOT_ERROR      No more disk units (increase the
MaxRemovableDrives setting in DBLSPACE.INI)

09H TOO_CLUTTERED_ERROR     CVF is too fragmented (increase the
MaxFileFragments setting in DBLSPACE.INI)
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DoubleSpace API Error/Return Codes