If you are getting fails with time machine after a few days/weeks/months then have a look at this link.
http://www.macfixit.com/article.php?story=20081030100706598
I found that if the drive is connected the back-up will not even start so this sounds entirely possible.
http://www.macfixit.com/article.php?story=20081030100706598
I found that if the drive is connected the back-up will not even start so this sounds entirely possible.