Are you backing up to a device - or a file? How is this being selected?

What Gail was trying to get at is that overwriting an existing file destroys any maintenance plan you setup. What good is a transaction log backup if it only contains the currently backed up transaction log? You won't be able to restore the system because you are now missing the previous transaction log backup.

If you are also overwriting the differential backup - you have the same problem. As well as overwriting the full. To understand better, what is going to happen if you have to restore the system to 10:50am, but your differential backup ran at 11:00am and overwrote the previous differential backup that was run at 10:00am? Add in the fact that you have overwritten the transaction log backups and you don't have a method of restoring to that point in time. The best you can do is restore to the latest differential.