I'm having the same problem in win xp pro that i saw on an old forum post that wouldn't let me reply for that very reason (something about no one being able to read a post that old or something). Neways, I recently upgraded to xp pro on my system, and when it first booted up, my 2nd (80 gig fat32) hdd had dissapeared. 98se was fine for years, but when I upgraded and installed xp, it lost the 2nd 80 gig hdd. So far i've tried resetting the bios (it detects both hdds just fine), and when i use a win98 startup disk, i can access all of the partitions on the "vanished" 80 gig drive. Funny this is though, in control panel > admin tools > comp manag. > disk management, it shows the western digital 80 gig drive as a 30 gig that's "healthy" and unpartitioned. Any ideas?

Try re-partitioning and formating the drive

under disk managment, right click on the drive and select mount volume.

Use the same disk management, and format the entire drive. xp will not recognize the drive if it is not formated.