Wd 2.0/3.0 issue

Hello all!  I have a 2TB 2.0 external usb, and a 2TB 3.0 external usb. Both are Mybook. For some reason, WinXP SP3 will not recognize both at the same time. Only one or the other. If I plug both in, neither will show up. Are they not compatible or conflicting? I don’t see anything in the device manager other than unknown device on the usb controller menu. Thanks for any help.

***Update*** I unplugged the second drive from the surge protector and plugged into the wall outlet. Seems to be working fine. Maybe the 3.0 drive was hogging all the power?

It could be a power thing they are sensitive. Sometimes with 2 drives of the same size you need to assing one a different letter and put it online.