This usually happens when you replaced a drive being backed-up with a different one, but with the exact same name as the original.  This appears to happen only on Leopard.

Because of the way Time Machine keeps track of drives, at one point it thinks they're the same, but later on realizes they aren't.

Either rename the drive (append "_2" or something), or delete all previous backups of it, via the instructions in Frequently Asked Question #12.

And note that Time Machine may do a full backup of the drive.  If there isn't much space on your Time Machine drive/partition, see the pink box in section #C4.  You may need to do item 2, 3, or 4 listed there.

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