Time Machine error code 21 and 19, "Failed to mount.... because the image could not be attached"

Question

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.

Answer 1

This turned out to be a keychain problem. Possibly I changed the NAS login password and never updated keychain.

1. Settings > Time Machine > Stop using "TimeMachine" for Backup
2. Keychain > search for NAS hostname and delete all matching passwords. I think the problem entries were in the System keychain, but I also deleted entries from the login keychain.
3. Settings > Time Machine > Add Backup Disk... > select "TimeMachine" > enter password

This situation was confusing because the messages from Time Machine didn't say anything about authentication, and Finder could connect to the NAS volume ok. But apparently backupd uses a different Keychain entry, which was bad or wrong.

Answer 2

My problem turned out to be that the Time Machine user account accessing the NAS had exceeded its quota from usage elsewhere in the volume (facepalm).

This ended up causing the Info.plist file inside the sparsebundle to be 0 bytes. Fortunately there was an Info.bckup file in there that I copied over to be the new Info.plist.

Answer 3

In my case (error 21) it was a garbled Info.plist file[1] (on Time Capsule). Fortunately there was a backup file called Info.bckup which I used to replace the garbled one. Which solved my problem.

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[1] Don't ask me how that stuff got into the Info.plist file...:

$ more Info.plist an Loop[2] : 50 Skin Side 3 Fan Loop[3] : 50 Skin Side 3 Fan Loop[4] : 0 0 44 271 47 0 0 0 0 0 0 0 0 0 Skin Side 4 Fan Loop[0] : 50 Skin Side 4 Fan Loop[1] : 38912 Skin Side 4 Fan Loop[2] : 50 Skin Side 4 Fan Loop[3] : 50 Skin Side 4 Fan Loop[4] : 164 198 0 0 0 0 0 0 0 0 0 0 0 0 HDD Radio Loop[0] : 100 HDD Radio Loop[1] : 102400 HDD Radio Loop[2] : 100 HDD Radio Loop[3] : 100 HDD Radio Loop[4] : 0 0 0 0 0 0 0 0 0 0 0 0 0 362 2G Radio Radio