Why does Time Machine hang/freeze on "Backing up..."? Lately, whenever I try to back up Time Machine to one particular external drive, it gets past the "Preparing..." stage (most troubleshooting documents talk about fixing problems with hangs in this stage). The status says "Backing up..." and the subtitle will say "Copying X GB of Y GB..." but it will make no progress even if I leave it overnight.

What's wrong?
I couldn't find any information on this issue, so I called Apple. After some digging, they told me that this could result from corruption in the OS X install. They asked me to reinstall OS X, which made me a little nervous, but when it was finished, I did a clean Time Machine backup and all went well.