Good points Deacon.
I have experienced similar problems with a drive first used on XP, then Vista (oh, the pain, the pain) and then on Win7. Win7 actually cleared up the issues when the drive was later connected back to the Vista machine.
The drive isn't 32 bit or 64 bit. It's just a drive. Assuming XP could read it, then Windows 7 should as well. Do other devices work on those USB ports? Does the drive work on another computer?
garthnathaniel: try going into Disk Management and removing drive letter assignments for all volumes on the ext drive.
exit Disk Management and safely remove the ext drive using the icon in the tray or right clicking the drive in Computer (eject [if present], then safely remove) physically disconnect the drive
restart Windows and login
connect the ext HD
you shouldn't have any drive letters assigned for the ext HD at this point
go into Disk management - maximize the window and post a snapshot
select the first volume on the ext drive and assign it a drive letter
- just the first volume for now (if there are more volumes, you can add them later)
post another fullscreen snapshot of your Disk management window
see if you can access the volume using the drive letter you assigned to it.
I don't fully understand the why and I'm not sure this will solve it. It won't hurt anything. There are more tools to try, but this is easy.
