USB hard drive lockup

All,

My Win XP desktop has 2 hard drives. The 2nd is a 1TB drive where I store
a bunch of things. PDF manuals, copies of install CD and DVDs, electronic
magazines, My Documents backup, Eclipse workspace backup, etc. If I want
some install CD I don't have to go scrambling to find it.

None of it is irreplaceable, but about 18 months ago I decided it would be
a real pain to recreate it, so I bought an external USB drive and, once a
month, I use Allway Sync to back up this hard drive to the external USB
drive.

Yesterday it wouldn't work. My PC just sorta locked up. Had an hourglass
mouse cursor, but I could move it around. I could slowly get my other apps
closed but couldn't reboot or shut down. I had to press and hold the power
button for 4 seconds.

So I tried it again after reboot before starting anything else. Same
result. Had to use the power button.

So I plugged the USB drive into a Win 7 laptop. Everything was fine.
Checked to USB drive for disk errors, none found.

This morning I tried it again on my desktop with no joy. Same result. The
only thing I can think of since last month is that last week I plugged in a
number of USB thumb drives to select one for Firefox portable (to get at
ASMI on a System i 520 if you remember that discussion). One of the thumb
drives appeared defective so I just pitched it, but that was before I
successfully set up Firefox, so at least that thumb drive worked after that.

Anybody seen this before? Ideas? Does Windows have some kind of "cache"
or something for remembered USB drives that's been hosed? I'm afraid to
plug in thumb drives at this point.

Thanks.