Adding a Time Machine destination fails with OSStatus error 17 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.
Digging through the console I found this telling error:

NetAuthSysAgent already_mounted: share &lt;TIMEMACHINE&gt; 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:


Connect to the share using Finder
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.
With the authentication prompt still open, go back to Finder and eject the share
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.