twIsDrive( nDrive, wSType, wFType )
 ----------------------------------------------------------------------------
     Get the status of a floppy diskette drive with the operation
     occurring in a window.


       Arguments:

     nDrive - 'N' Drive number. 0 for drive A:,
                                1 for drive B:

     wSType - 'N' TSDWIN Shadow type. Defaults to current shadow type.

     wFType - 'N' TSDWIN frame type. Defaults to current frame type.

       Return:

     Return messages depend on the situation found by DOS:

     "Drive Door Open or Diskette inserted upside down"
     "Diskette is unformatted"
     "The Diskette in Drive (A/B) is Write protected",;
     "An Undetermined Drive Error has Occurred!

       Example:

     twOpen()
     twOpen( 2, 5, 21, 74, "w/b,gr+/r,,,n/w", 3, 4 )
     twTitle( "Testing twIsDrive()" )
     DO WHILE .T.
     twSay( 2, 5, "Enter Drive to Check!" )
     @ twRow( 4 ), twCol( 12 ) GET nDrive PICTURE "#"

     READMODAL( GetList ); GetList := {}
     IF LASTKEY() == K_ESC
             EXIT
     ENDIF

     twInfo( "Return Value: " + STR( _twIsDrive( nDrive )),,,,,;
                                        3, 4 )

     ENDDO
     twPop();twPop()