Time Machine backups stuck on "preparing backup"

Google is your friend:
http://www.google.com/search?q=Time...s=org.mozilla:en-US:official&client=firefox-a

Eric052492 said:

Every time I turn my time machine drive on, it gets stuck on preparing and eventually gives me the message that the backup failed. When I try repairing the disk with the disk utility, it says the drive cannot be unmounted. Any solutions to what might be wrong?

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The "preparing" step can take many hours. Best to let it run over night. It has to copy everything to the external drive. If the external drive is not large enough the backup will fail. Also if the drive is a problem (bad sector on the drive) the backup will fail. Check logs to see why it failed

Lastly, you have to un-mount (eject) the drive before disk utility will work on it.

when/if you ever unplug the TM drive, be sure to unmount it correctly. If you just yank out the USB cable, then it can cause some corruption on the drive and it'll take forever for the backup to 'prepare' the next time its plugged in.

That's just one thing I've noticed that causes the "Preparing Backup" message to appear; might not be your case or not. Just throwing it out there.

My drive has FW800 and that sped things up considerably. I would think that the interface you are using and the amount of data is contributing to your issues.