I can not get Time Machine to back up an external WD Passport 2 TB drive! Has anyone else had this problem? The drive is new and I was running Snow Leopard and upgraded to Lion and it still won't do it. Time Machine backs up the internal drive fine.

I bought 2 2TB WD Passport Drives with the intention of housing my iTunes/iPhoto libraries off my older MacBook. I have successfully transferred the Libraries to one and am using the other one for Time Machine. In Snow Leopard I could back up my computer to Time Machine no problem but when I did not exclude the iTunes/iPhoto drive the back up fails. Both drives have been reformatted, permissions repaired and checked in Disk Utility and one was replaced. I have been to the Genius Bar now 5 times. I have tried using USB drive and FireWire. Both drives are recognized and are working properly otherwise regardless how they are connected. Finally yesterday the Apple store installed Lion to see if that fixed the issue and it did not. Time Machine successfully backed up a USB Flash drive, and today I will try an older external drive. Any ideas on what else to try??? Do I have to resort to third party software like Carbon Copy Cloned to get this done? Any ideas why this wont work? I am using a FireWire converter but as I said, both drives are working normally otherwise (libraries working, time machine working for the computer's hard drive. Any ideas would be appreciated!!?