A simple, Win32 specific implementation would be to check for the nul file. Every drive and directory has a nul directory associated with it. So to see if a drive/direcotry exists, check to see if the nul file exists.

print (system("IF EXIST J:/nul exit 1 ELSE exit 0\"))
          ? 'drive exists'
          : 'drive does not exist;
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Interestingly enough the perl operator for file existence (-e) doesn't identify the nul file so I guess it is meaningful only to DOS/Win32 OS's