Compatibility: 6.0+
Expects: AX    4a11H (mux number)
BX    0006H (DoubleSpace fn code)
DL    compressed drive number to unmount (0=A, 1=B, etc.)
Returns: AL    DoubleSpace API Error Code (00H=success)
Info: This unmounts a compressed volume; severing the connection
between a compressed volume and its drive ID.

It's probably a bad idea to unmount an drive on which files are
open (for instance), or anytime Windows is running.

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INT 2fH 4a11H BX=0006H: Unmount Dblspace Drive