Time Machine stopped working after upgrading to Yosemite I have upgraded my Mountain Lion system to the latest Yosemite release in the App Store.

When I click the Back Up Now link in the Time Machine menu it says it's preparing the backup but it never gets past this and actually starts backing the system up.

How do I get Time Machine working again?
I had the same problem.  At the Apple troubleshooting site, it stated that the initial backup after an upgrade will take a long time.  I hit "Back Up Now" before bed, and when I got up in the morning, it had completed the Back Up.  After the initial Back Up, it works just fine now!