The problem for me was a faulty USB cable. I have several WD Passport drives, so I used a cable from a different one and it came up and was recognized correctly.
I had gone through a great deal of trouble shooting during an earlier attempt to connect this drive, and had begun to suspect the cable only after I had finally (somehow) succeeded in getting the drive recognized, and was saving data, and I moved the drive a little and got an error message. So I inferred that the cable connection at the Passport was delicate.
A long time later, I wanted to save more data to that same drive. It wouldn't recognize either on XP or Vista (I tried two different machines), giving me an Unknown USB device error, so I wondered whether the cable might be the problem. As described above, I used a different WD cable and it was recognized.
BTW, I use Acronis WD backup, downloadable free if you have a WD USB drive) at
which can generate a boot CD for a cold backup (the best way, in my opinion). There are also instructions on the web at
for creating a cold boot thumb drive or USB hard drive, in case your notebook doesn't have a CD drive but will boot from USB.