Get an array of available drive letters
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 Syntax

        H_GetDisk()

 Arguments:

      None

 Returns:

      An array of available drive letters

 Description:

      Tests each possible drive letter and returns an array of those which
      are found to be valid

 Examples:



 Header File: NANFORUM Toolkit

 Source: GETDISK.PRG

 Author:  Martin Colloby