Time Machine can't use Disk on MacMini for backups; why?

I solved it!!!

It sounds obvious when I say it like this: When you try to set up a Time Machine backup across the network to a remote disk, you must use the exact same credentials to access that disk as you did to make it visible in the Finder.

If you don't you get the message I reported above.

Why would you use any other credentials? Because at some point recently, the possibilities changed, and now when the Finder connects to a remote disk, unless instructed otherwise, it does so using some variant of your Apple ID. So, when you try to set up the Time Machine connection to the remote disk, it even tells you that that's the name it's using and asks for the password, but the Apple ID password doesn't work.

So, you try your own id and password, and that satisfies Time Machine, in that it allows it to see the remote disk. But the moment it actually tries to use the disk it gets blown away because the credentials aren't the same.

This only affects new Time Machine connections. Once one is made, from then on, Time Machine can use those credentials without any need for them to match what's happening in the Finder.

Phew! This was driving me nuts. I was on the phone all afternoon yesterday with a senior Apple Advisor and he was stumped, too.

Dave