Can't unhide drive. Drive letter conflict, drive disappeared.

I added a hard drive. Used Disk Management to map it to O: and P:
When I plugged in my flash drive, hard drive O: disappeared. (I think I
mapped the flash drive to L: M: N: and O: previously but forgot about it)

I checked the hard drive with Partition Magic 8 and O: is there but
hidden. I tried to "unhide" drive O: a few times but each time I
reboot to XP, it is hidden again.

If I right-click on the hidden drive in Disk Mangement, the only
option is "Delete partition". "Change drive letter.." is disabled.

I remapped the flash drive to I: J: K: L: but hard drive O: is still
hidden and P: is visible.

Is there a way to restore O: to visible?

TIA