Ever since upgrading to Sierra, I keep receiving the following notification from TimeMachine: "Backup has failed. The backup disk image "/Volumes/Data/Name.sparsebundle" is already in use." I am trying to backup to a WD MyCloud NAS. I have tried the following: Restart computer and NAS Remove and re-add NAS drive to TimeMachine Delete previous TimeMachine backup from NAS and start a new one Removing and then adding the NAS drive back to TimeMachine fixes the problem for a few hours, but then the notification comes up again. Any advice?