My Time Machine backup stopped doing automatic backups. If I start a manual backup, it works fine, and it schedules the next backup. But when the time comes, nothing happens. It does not even try to do a backup. I am using Yosemite (10.10.5) on a late 2013 iMac. I have tried deselecting and unplugging the external backup drive, throwing out the com.apple.timemachine.plist file. Then I reselected the external drive. This made no difference. Any help would be appreciated.