I had a major bummer experience – I have Time Machine running off a Time Capsule.
I noticed (eventually!) that my backup hadn’t completed for days. Eventually Time Machine popped up a dialog telling me this as well. But why? I watched it run and saw that it never got out of the “Preparing” stage. I let it run for hours. Trying to be especially patient, I even let it run for 48 hours. It never stopped “Preparing.”
I read online you could read some logs or something using Console.app. Console’s icon bounced a couple times, but never brought up a console. I had to Force Quit it. And still Time Machine was Preparing.
I called Apple support. I have an Apple Care membership. They suggested I load the Time Capsule as a drive, and delete the “sparsebundle” there. This would erase all my backups and history, so I could start fresh. While this may have solved things, I didn’t want to do this: what would be the point of backup with Time Machine then?!
They gave me one more hint before I had to go. They said to use Disk Utility and attempt to Repair Disk on the Time Capsule volume. So I did… the process took a long time, even though it estimated it would take an hour… it ended up taking over 2 hours. I let it run all day.
When I got back some bad permissions had indeed been detected, and repaired. I shut down Disk Utility and attempted another Time Machine backup.
This time it worked! It did Preparing for only a few minutes, and then completed the backup, adequately proving that I hadn’t started over from scratch with no history — if I had, this Preparing would have taken hours.