WD Passport Essential & Time Machine - disk unmounts in the middle of back-up

When I first bought the Passport Essential (Dec 2011), I installed the Mac software and used it to back up with Time Machine no problem. I have not been able to again since. Each time I attempt to back up it begins and within a couple minutes stops and TIme Machine says it will next back up when it finds the disk. Yet, the disk is still attached. 

As per the advice of several forums, I reformatted the disk with disk utility, made sure there was only one partition and tried again. No luck. I have formatted with the SmartWare software. No luck. 

Anyone have any advice? Does this drive not work with Mac? The first time it was used, when it worked, I was using Snow Leopard, but I attempted to do a follow up back up with Snow Leopard, Lion and now Mountain Lion. None of these have worked.

Help!!! So frusterating. 

Have you tried using the drive in a different computer? And also using an additional USB cable? Do you have the same issues if you transfer the files manually instead of using Time Machine?

nother box keeps trying to tell me I can’t open the VCD and have to download up to date firmware- I thought the beauty of it was that it downloaded updates on its own. It seems you can manually type in password each time you want to do a backup which again defeats the object of a set and forget