With macOS sierra (10.12.4), Time Machine backups to a Synology NAS are failing in the "preparing backup" phase. The UI messages aren't helpful, but console shows this error from backupd:
Failed to mount '/Volumes/TimeMachine/fubar.sparsebundle'
because the image could not be attached, error:
Error Domain=com.apple.backupd.ErrorDomain Code=21 "(null)"
UserInfo={MessageParameters=(
"/Volumes/TimeMachine/fubar.sparsebundle",
19
)}
This repeats every 60 seconds for several minutes, and then Time Machine gives up.
I'm familiar with the sort of problem fixed by https://apple.stackexchange.com/a/39842/30668 but this seems to be different. I can mount the network volume and attach the sparsebundle. Running fsck shows no errors. The sparsebundle plist shows VerificationState
0, and doesn't have a RecoveryBackupDeclinedDate
.