XPlastDisk()
   Return the last Logical disk in the system
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   Function:   XPlastDisk()

               Returns the last Logical disk in the system, all
               disks between 0 (ie. A:) and XPlastDisk() are
               available disk drives at this moment, this
               includes any possible Virtual or remote disks.

               Note that not all disks between 0 and XPlastDisk() are
               available by definition. XPlastDisk() in effect returns
               the highest possible letter set in Config.sys (LastDrive).
               Some of these drives may or may not be used (eg. By DOS'
               SUBST command).

   Syntax:     XPlastDisk() --> nLastDisk

   Arguments:  None

   Returns:    A number, 0=A etc..

   Usage:      @ Say "Disk#:" Get Disk range 0,XPlastdisk()