Time Machine stopped working

I want to back-up my iMac before I update the MacOS to 10.12 Sierra.

Something weird happened today: I plugged in my external hard disk to backup my iMac, told Time Machine to "Backup Now", and all the Time Machine status says is "Preparing Backup..." I let the iMac sit for an hour and nothing happened. I skipped the Backup, turned Time Machine off, then turned it back on again, and tried to initiate a new Backup. Still "Preparing Backup..."

最后更新：2017-09-21 05:35:09