I'm trying to add a specific, external server as a Time Machine destination. It's not discoverable via Bonjour (always at least, I want to use time machine backup over the internet) so to get it to show up in the Time Machine preferences I have to connect to it. I can then add it from Preferences. The weird thing is if I try and add the other destination which is visible via bonjour it works just fine, despite being able to mount the share normally in Finder.

up vote 28 down vote accepted

Digging through the console I found this telling error:

NetAuthSysAgent already_mounted: share <TIMEMACHINE> already mounted

TIMEMACHINE is my share name in this case. As the error suggests, Time Machine is trying to mount the share again which it can't do because it's mounted already. But wait! If I disconnect from the share will leave my list!

Here's where some weird UI trickery comes in:

  1. Connect to the share using Finder
  2. Open Time Machine preferences, click 'Select Disk', and then double click the disk at your desired address (in my case I wanted my FQDN). You should now be at the authentication prompt.
  3. With the authentication prompt still open, go back to Finder and eject the share
  4. Sign in to the waiting prompt

and the disk should be added! I've never had this issue before but it's come up on High Sierra which as it turns out has some really bad AFP/SMB bugs.

  • Which is the log file with the NetAuthSysAgent error? I run into a similar problem: apple.stackexchange.com/questions/321069/… I can add the disk as a TM target using your solution, but TM won't start the backup. – JYF Apr 1 at 11:13
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    After reading this comment I had success with simply ejecting the shared volume in Finder and then back to Time Machine to select it as the backup disk. What this means for future backups is currently unclear, but it appears to be at least a shortcut to here ^^. – JGrubb Apr 15 at 13:46
  • fwiw - i was connecting to a macmini with a passport drive connected to the macmini. it is currently "preparing backup" -- i expect to have success and replaced my failed local timemachine for my networked time machine. – mobibob Jun 2 at 20:50
  • Works. What a crappy bug – Andi Giga Jun 26 at 15:10
  • This took me a while to figure out the exact cause and this help, so thank you. In my case I normally log into my NAS as "Guest" and then let Time Machine Server use "Jon". I had logged into the NAS as "Jon" in finder for a while and that connection was blocking TM. – jb510 Aug 12 at 18:39

protected by Community Apr 15 at 3:14

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