Mac – Network share always mounting in Finder, but never for Time Machine (High Sierra 10.13.4)

file-sharingfinderhigh sierraNetworktime-machine

I have two Macs (MBP 2017 and an older Mac Mini with two internal SSDs) on my local network, both running macOS High Sierra 10.13.4, and I'm encountering strange problems.

One of the Mini's internal SSDs (HFS+) is partitioned. One partition is shared (using File Sharing in System preferences) in order to be used as a Time Machine drive for the MBP.

Two problems:

On the MBP, the Finder will mount the Mini's network share without asking any credentials, even if I forbid access to any user (on the Mini, while keeping the share enabled). Why? And where are the Mini credentials stored on the MBP? I'd like to erase them but I can't find them in the MBP's Keychain utility.
Even if I recreate the share from scratch on the Mini (which recreates users for remote admin access, etc.), Time Machine on the MBP fails to mount the Mini's share. Specifically: I can enable the network share as a TM drive, but TM will complain it can't start the backup because of a credentials problem, even if the share is already mounted by the Finder!…

The problem seems to have appeared with the 10.13.4 update. I had similar setups with previous macOS versions that work flawlessly.

I'm puzzled. Any help really appreciated.

Best Answer

Problem solved. Here's the reason of all this:

Both Macs were configured with the same iCloud account.
For some reason, the admin user on the Mac Mini had an alias that was the email address of the iCloud account.
The MBP's Finder mounted the network share using that iCloud email address (admin alias). That's probably why it didn't ask any credentials.
But something went wrong with the MBP's Time Machine process, that kept trying logging with the Mini's admin user "real" credentials (not the iCloud alias).

What I did:

On the Mini, in the "Users & groups" preference pane, I right-clicked on the admin user and deleted any aliases found in the advanced options. See this for step-by-step instructions: Prevent File Sharing access with Apple ID
On the MBP, I mounted the network share (using the "real" Mini's admin credentials), then I recreated the Time Machine setup.

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You can set up your iMac as a fully functioning time machine server by purchasing the OSX Lion Server from the Mac App Store.

This provides you with an official method free from hacks to allow you to backup individual machines such as macbooks etc to a single target machine. I backup my Macbook Air to my iMac in this way. For extra safety, I occassionally clone off this backup volume in the iMac to an external disk and store it out of the house.

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I think I can get you started, but using terminal commands, which you said you'd prefer to avoid. Apologies, but it might give you a starting point. All of the following you could put in a bash script and run as a login item.

You'll need to first share out the target drive(s) (MyBook in the examples below) using file sharing (System Preferences > Sharing > check the File Sharing box, add the drives you need and set appropriate permissions - I just use Everyone to keep it simple).

You can mount a remote machine and/or its associated drives using mount like so:

Make a directory as the mount point (obviously call test whatever you want):

mkdir /Volumes/test

Now mount the remote drive. I'm assuming this is already connected to your iMac and appears when you connect to your iMac using Finder:

mount -t afp afp://<your mac's name>/<drive name to mount> /Volumes/test

On my remote Mac Mini, to connect to my MyBook attached to it via USB, that would be:

mount -t afp afp://bobs-mac-mini/MyBook /Volumes/test

If you need to authenticate (I got error -5000 when trying to mount my home folder), you can also do this using

mount -t afp afp://<username>:<password>@<your mac's name>/<drive name to mount> /Volumes/test

However, the password would unfortunately have to be in the clear. So again, this might be:

mount -t afp afp://binarybob:password123@bobs-mac-mini/MyBook /Volumes/test

You can also connect to your home folder using the above method, just by replacing MyBook with the name of your home folder.

The drive you mounted should now appear in the finder and you should be able to use it like any other locally mounted drive. When you're finished, you can do:

umount /Volumes/test

to remove it. BTW, if you're not an administrator, you might need to add sudo in front of each command and type an administrator password.

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