Extract from http://support.microsoft.com/kb/310123
Windows cannot load the device driver for this hardware because there is a duplicate device already running in the system. (Code 42)
A duplicate device was detected. This error occurs when a bus driver incorrectly creates two identically named children (bus driver error), or when a device with a serial number is discovered in a new location before it is removed from the old location.
Restart the computer.
On the General Properties tab of the device, click Troubleshoot to start the Troubleshooting Wizard.
What you could try is to use a different type of USB-port for the two disks. One on the back, one on the front. Or one on the mobo or one on a PCI-USB card. That might be different enough to be not the same.
If that doesn't work, try exchanging one of the two for a different make/model if you prefer to have these disks external. Internal, of course, is much faster, so you must have a good reason to use them as external disks.
Hope this helps.
I just received a second WD 500GB, both made into externals by someone on eBay. Having booted it 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 --
"Windows cannot load the device driver for this hardware because there is a duplicate device already running in the system. (Code 42)"
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:
400GB second added
My brain's already more-or-less asleep, so this will have to do. I'm expecting and hoping for a relatively simple fix. Please let me know if I've omitted any relevant info.