Time Machine failing on Snow Leopard

Hi all

For some weeks now Time Machine has been telling me that it is unable to back up because there isn't enough space on the allocated hard drive. I am on OSX 10.6.8 and am backing up to a 500GB internal hardrive, which I appreciate isn't large (I am a photographer and shoot around 20-30GB per job), but I regularly remove the work to two other backups to keep the main hard drive (that I'm backing up with Time Machine) free for current work only. I have now excluded almost everything from the backup apart from my images, my documents and the library - a total of only about 400GB according to the information on folder sizes. And I have even completely cleared the back up hard drive using Disc Utility, and renamed it. Yet it is still refusing to back up. It is telling me the following:

Time Machine Error. This backup is too large for the backup disk. The backup requires 509.65 GB but only 499.32 GB are available.

Yet it has estimated itself that the backup is only 450GB, (even though the folders are actually only 400GB in total).

It's almost as though Time Machine needs its own cache removing - but nothing I do seems to work.

Anyone come across this before?

Thanks for any advice.

Rob