Storage: Can't add 2nd external 500GB

Hi all,

I just received a second WD 500GB, both made into externals by someone on eBay. Having booted the computer with the first External for months, never a whisper of a problem.

Now I try to attach (so software/drivers needed for XP evidently) the 2nd 500BG unit, being identical to the first in every way I can tell. Swapping the one which boots up alone, and fine, several times seems to indicate that both of the drive's are fine.

But when I try to boot with both drives plugged-in, the computer fails to recognize
either drive.

The problem appears (I'm hoping) fairly simple -- obtained from the Device Manager's Universal Serial Bus Controllers/USB Mass Storage Device (there are 2, but the first doesn't seem to be performing any actual function.)

"Windows cannot load the device driver for this hardware because there is a duplicate device already running in the system. (Code 42)" http://support.microsoft.com/kb/310123

Surely this can't be an uncommon problem, but a few hours of Googling and forum searches got me nowhere. I suppose I could run one of them from the preexisting SATA port in order to get away from the USB, but this sounds like a rather harsh solution to what must certainly be an easier fix. Maybe something as simple as slave jumper settings?
(Which I know nothing about, but we learn what we need to learn.)

BTW, here are my specs, in case any of it matters:

Dell 410XPS
3GB RAM
Internal drives:
160GB original
400GB second added


Cheers,
Michael