From the Western Digital BBS:

FASTDISK drivers for Caviar drives/Win 3.1 to allow
32-bit file access (540HD owners will want this).

Originally called WIN31.EXE on the WD BBS, which
can be reached at 1-714-753-1234 (8N1, 1200-14.4k).

This made my Gateway P5-60 w/Caviar 540 scream...