Time Machine (or ext HD not backing up)

For the past few days, Time Machine has not been backing up. The external 500 gig hard drive is making a bunch of noise as if it is backing up, but it just does it forever and under SysPrefs, it says that it is "preparing" to back up, even though it just does it forever. I've tried restarting and stuff, with no result. Retreiving files from previous backups works fine using TM. Any idea guys?

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Anybody?

edit - nobody cares, but I have since solved the problem. I tried several things at once and one of them must have fixed it (it just needed to "prepare" for quite some time). But it works fine, so no worries.