Network share always mounting in Finder, but never for Time Machine (High Sierra 10.13.4) 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.
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 &amp; 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.