RF>  I'm making a DOS program that will run under DOS and OS/2.
 RF>  The only problem I have is that the information about the
 RF>  cluster size of a HPFS partition always comes wrong... 

Probably because HPFS doesn't have clusters.  (-:

 RF>  So, does anybody knows how can I identify a HPFS partition
 RF>  from a DOS program running under OS/2 in a window or full-screen 
 RF> session?

HPFS has "." and ".." entries in the root directory. 
FAT does not.

This is not foolproof, because other filesystems, such as LAN Server and
CDFS, also have such entries.  Basically this is a FAT detection method,
rather than an HPFS detection method.

 ¯ JdeBP ®

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