If you get the error message "Time Machine could not complete the backup. The backup disk image “/Volumes/mac-backups-1/machine-name.sparsebundle” is already in use." and you are using a Synology DiskStation DSM 5.2 (NAS) as the destination for Time Machine backups, the following steps resolved the issue:
- Login to your DiskStation as admin
- Select the Options icon (at the
top right -- looks like a person's head)
- Options > Account > Account
Activity > Connected Users
- For each user on AFP Service and
Resource: mac-backups, press disconnect (red circle with white
horizontal bar). The issues seems to be that there are two or more
connected users both connected to AFP > Resource: mac-backups.
There should only be one.
- Click Refresh. There should now only be
one user connected to AFP > Resource: mac-backups.
- On your MacBook > Time Machine icon > Start Backup. Time Machine backup begins (circling counterclockwise) and you no longer get the above error message.
I am not sure exactly how this situation occurs, but here is my theory: After disconnecting the MacBook and reconnecting to another network (moving my MacBook to another location while traveling), the connection to the synology resource: mac-backups (used by Time Machine) did not get properly disconnected, so when the machine was reintroduced into the network, another new connection was established, even though the existing connection was still present, creating a conflict and the subject error message.
To Be Tested: To prevent this error from happening again, if Time Machine is in-progress doing a backup, try doing a "Stop Backing Up" command in Time Machine before moving your MacBook to another location off-network. Hopefully, when you bring the machine back to the network where the synology NAS is located, the error should no longer occur.