INT 21 - DOS 5+ - DISABLE DRIVE                                                 
	AX = 5F08h
	DL = drive number (0=A:)
Return: CF clear if successful
	CF set on error
	    AX = error code (0Fh) (see #01680 at AH=59h/BX=0000h)
Notes:	simply clears the "valid" bit in the drive's CDSsee Current Directory Structure
	this function is not supported by Novell DOS 7 through at least
	  Update 4, but support was added by Update 13
SeeAlso: AH=52h,AX=5F07h"DOS"