Mac – Time Machine error code 21 and 19, “Failed to mount… because the image could not be attached”

backupnastime-machine

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.

Best Answer

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

Settings > Time Machine > Stop using "TimeMachine" for Backup
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.
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.

Related Solutions

Mac – Repair Time Machine sparsebundle that will no longer mount

I have a write up on how to try and repair NAS-based sparsebundle errors on my blog. In summary:

hdiutil attach -nomount -noverify -noautofsck /Volumes/{name of your disk}/{name of}.sparsebundle

You will then see something like
```
/dev/diskx Apple_partition_scheme
/dev/diskxs1 Apple_partition_map
/dev/diskxs2 Apple_HFSX
```
Where x is the disk id for the external disk. x might be 2, 3, 4 or higher. You are interested in the one labeled Apple_HFSX or Apple_HFS.
fsck_hfs -drfy /dev/diskxs2 using whatever relevant device was located in Step 1.

Hopefully, you will eventually see

The Volume was repaired successfully
hdiutil detach /dev/diskxs2

However, since OS X 10.6.3, Time Machine will refuse to write to a destination volume that fails its verification. Even if the process above succeeds in recovering the backup, you may still need to remove the black marks that Time Machine wrote when it failed verification.

Unlock the sparsebundle

chflags -R nouchg /Volumes/{name of your disk}/{name of}.sparsebundle

Move it back to its original location

mv /Volumes/{name of your disk}/{name of}_YYYY-MM-DD.sparsebundle /Volumes/{name of your disk}/{name of}.sparsebundle

At the top level directory of the sparsebundle, edit the file com.apple.TimeMachine.MachineID.plist.

Remove

<key>RecoveryBackupDeclinedDate</key>
<date>{whatever-the-date}</date>

Change

<key>VerificationState</key>
<integer>2</integer>

<key>VerificationState</key>
<integer>0</integer>

Mac – Time Machine error – backup disk image could not be created

Try to setup Time Machine from scratch:

Turn TM off

Turn it back on. Select a disk (the Time Capsule should show up ok).

You should get a message that TM is starting in 2 minutes..

If that has problems, can you post here again..

edit 1 was an issue with computer name needing to be set (see in comments)

Best Answer

Related Solutions

Mac – Repair Time Machine sparsebundle that will no longer mount

Mac – Time Machine error – backup disk image could not be created

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