Time Machine Won't Recognize External Hard Drive

I did a search before posting it, but couldn't find the answer to my question.

I am wondering if it is common for Time Machine to stop recognizing a hard drive. My Western Digital My Book hard drive stopped being recognized about two months ago. I finally decided to buy a new one which I connected. It is backing up fine right now with time machine. I am suspecting that one of the reasons it got messed up to begin with is that the cord got pulled by accident and it was disconnected without being properly ejected.

A few days after replacing my hard drive on my iMac, I was using my husband's iMac and noticed that his computer has not been backed up by time Machine in over 30 days. He also has a Western Digital My Book. It is not being recognized by Time Machine either.

I find it hard to believe that both of these drives failed.

I have a Mac Book and I am thinking of plugging the drives in to the Mac Book to see if they are at least recognized. I also have a Tablet PC and a Desktop PC and may try the drives with those computers also.

I was simply wondering if it is a common problem for working drives to fail to be noticed by Time Machine. They don't even show up in the Finder. So they aren't being recognized by the computer at all.

Any thoughts?

Thanks in advance.

Terri

Additional Information

I checked my husband's iMac. His backup disk is definitely NOT being recognized. I disconnected it. I then connected it to my Mac Book. The Mac Book can see it and even knows that it is a Time Machine back up disk.

I also hooked up my own desktop backup disk to my Mac Book using the usb connection rather than the firewire connection I had been using on the desktop. It was also recognized by the Mac Book and the contents are there, just as they were on the other back up drive.

I am wondering if I should simply reformat these two drive using the Mac Book and see if they will then be recognized by the desktops.

I suppose I could call Apple Care and ask about this as I can prove that the disks still work because they work with the Mac Book. It looks like a Time Machine issue.

Terri

I'm having this exact same problem. Did you ever find out how to rectify it? Anyone else know?