No consolation for you, but I have the same problem.
USB mouse & keyboard work fine.
self powered USB hard drive won't show in explorer.
Device manager reports both the USB port and the hard drive as working
correctly.
Disk manager shows it as 'not initialized'.
Plugging into other PCs the USB hard drive works fine.
Took the drive to my PC makers local workshop and plugged into identical
model straight out of the box and all was fine.
Went home and removed all peripherals and had success.
Main culprit seemed to be a pcmcia wireless network card. Toshiba laptop
seems to see the slot as a potential hard drive for one of their own
accessories so I thought that was that. Unplug the network card, reboot,
plug in the USB drive and away i went.
For about 4 weeks.
Now nothing can get explorer to see the drive. I assume this is because the
disk management system doesn't recognise the disk correctly, even though XP &
2000 on three other brands of PC recognise it instantly.
My advice is
1st to plug into another XP box just to be sure your USB drive is working and
2nd unplug all other devices and try your USB drive after a reboot.
I'm close to rebuilding my PC to try to get success, but of course I can't
back it up anymore so am not keen to start from scratch with a stack of CDs.
Keep us posted of your trials and successes (hopefully).
Seems what I thought was a Toshiba specific problem might be more widespread
than expected.