And for those of you that enjoy the nitty-gritty blow-by-blow technical fine points, here is my story....
A month ago, my 2007 MacBook Pro gave the first hint of entering the death spiral. I stopped all life activity that night and immediately ran a complete new Time Capsule using Time Machine. It was successful.
Unfortunately, I put it on an external hard drive that has shown a little flakiness as I have several 'rugged' drives that look identical. I knew I had another Time Capsule that was about 4 months older that would work if necessary.
I kept using the '07 MBP trying to get it far more cleaned up and ready to make a compact, easy-to-migrate Time Capsule. I could get an hour out of the machine most days. (I think the heat sink thermal paste on the video card is failing and there is little point in repairs at this point. I got a great ten year run on it and had no complaints).
But I never quite finished that and eventually the '07 wouldn't boot up, some days. Along the way I discovered a used mid-2012 MBP for sale via consignment at a PC repair shop, the increasingly famous good-ole "Model 101" for a good price with what looked to be very light use by an owner who had purchased it from my local high school (it appeared they had done a very professional sale of the machine, probably via a clean install). The shop had the admin account password and I even discussed the details with the seller. He claimed to have "never" used it and this seemed to be the case, with no email, photos, music, or even any documents of any kind present on the machine.
So at the shop I changed the admin account password and changed the user name to my user name on my '07 MPB. (That was my first mistake, do not do this).
I decided before migrating my data from the '07 machine's time capsule, I might as well 'start fresh' with an upgrade to High Sierra, as I had been using Mountain Lion. I don't even remember which OS was on the purchased machine. Along the way I discovered the seller had used the machine enough to wrap an Apple ID into it, and another email address into FaceTime somehow.
I attempted my first migration. It did not go well. macOS does not like the idea of re-using an exact admin account name. (The UID501, which I have never read about anywhere else). I think I went with the option of creating a new user account to host the migration; perhaps Apple implies/hints that can be temporary, I'm not sure.
The migration failed part-way through. Probably due to a bad sector on the external HD that had my most current Time Capsule on it. But I couldn't tell for sure as there are many reports of migration failures, perhaps a few more just lately with High Sierra, not sure.
By this point I was regularly running into issues with calls to the seller's original Apple ID and sometimes his FaceTime account. I decided to try and erase all remaining vestiges of his ownership from my 'new' (5 year old) laptop.
I Googled up some instructions and discovered that user accounts can be edited in more detail by a certain key-click combination. I won't share it here - you don't want to go down that road - I'm warning you ....
I changed the "shortname" on the admin account, because it was the seller's full name. DO NOT EVER DO THIS IN MACos. WHATEVER YOUR GOAL IS IN DOING THAT, THERE IS PROBABLY A BETTER WAY TO ACCOMPLISH IT, I PROMISE YOU!
There was some verbiage on the screen about changes not taking effect until after a Restart, so I immediately summoned the nuclear football from my 24/7 military aide, and pushed the red button. (The one marked 'Restart' on the machine). Perhaps if I had let the UNIX kernel somehow work on my desire to change the shortname, there is a way for that to actually work out. But I doubt it. UNIX is fine with a single, permanent, shortname for what is almost, kinda-sorta, the root account, so why would some idiot want to change it? UNIX doesn't need that, so why should you. DO NOT DO THIS.
The restart went through and the smoke cleared. I could now use my 2012 MacBook Pro to my heart's content - as a "Standard" user. There was no "admin" user. No matter what I did. There was the intriguing "Other User" option that I had never seen before, so I could type in all kinds of usernames and passwords there - mine, the original owner's, my original account on the '07 machine, all of the passwords, in every combination. Nothing worked. I no longer had admin level control of my machine, and all of my OS X life going back to Cougar or Panther or whatever, was locked in a Time Capsule.
Enter Apple's most recent and nearly most infamous "Bug" - I had High Sierra 10.13.1.0 on this machine - I could be "root" with no password at all! What a relief! I was blown away by the news of this bug, and even more blown away that it came along just exactly perfectly right when I needed it. I could control my machine just like a Swiss Watch.
Sort of. It was a fun little bug. I am more than a little tempted to someday create an account called "root" just to see it up there on the task bar again, but I now know well how many problems that would probably cause. DO NOT DO THAT. I think it gave me a little insight into how the bug came about - it was incredibly handy to just type 'root', hitting Tab, and then Return, into any permission dialog box that popped up and my guess is a programmer working on testing High Sierra set up this little back door during some pre-public-Beta phase and then forgot to ever close the door later. But we will never know.
Oddly, Safari did not work for 'root' at all - it told me Apple.com had a suspicious certificate! That would have been a classic screenshot, if only shoulda/coulda/woulda. (It actually reported that for every single web address).
But I could still use Safari normally in my 'standard' account. So off I went to go through all the steps to create a bootable macOS installer USB drive. Getting the High Sierra install app to download correctly was an adventure - if you don't see it give a running time estimate right under the 'download' button on the pretty splash screen, it is probably downloading stuff and setting up an alias that you can't use - 'the 19 megabyte version'. But a lot of something stuff really did download, I watched it happen via Activity Monitor...
I finally got the installer app in to the Applications folder correctly, at 5-point-some Gigabytes, as it's own stand-alone little app. It had even launched correctly when the download finished - if you don't see that happen, do not pass Go, waste 200 more minutes and start over. (throttled public wi-fi access points for me in my life, ugh).
I was just reading the instructions on creating the install USB when my '12 MPB informed me it had downloaded a High Sierra update and I should click 'Restart' to continue. OK, makes sense.
NOOOOOO! Not now! I would lose my awesome power of ROOT! I can't remember if I had an option on that restart, but it was water under the bridge. I did not suspect I would need admin level access just to create a bootable USB drive, but them's the rules, it turns out.
'root' was a distant memory now. I was back to being only 'standard'. My '07 data was as far away as ever.
I continued my research - thankfully I had an iPhone with me. A strange thing about researching these issues is that Google will show you results from forum threads inside the Apple Developer community - that I couldn't access. I don't know why Apple lets Google scrape those forums, doesn't quite make sense. Perhaps with more persistence I could have eventually read those threads, but I didn't feel like that should be necessary and there would be a good chance a lot of it would refer to conditions in the Beta of High Sierra.
There are a variety of techniques to recover passwords, restore hosed machines, etc. out there for macOS. I changed the password on the one account I did have via my Apple ID. That didn't help - it was still a 'standard' account.
I discovered the idea of deleting AppleSetupDone, tricking the machine into allowing a new admin account to be set up.
That worked for me. Now I finally had an admin account. I created a High Sierra installer, wiped the hard drive and set up a new clean High Sierra machine. Man that picture of the High Sierras makes me want to go Trout fishing - there is one hiding in the stream likely flowing through that draw on the left, just under those conifers, I can feel it.
But I still had the problem that migrating in my '07 Mountain Lion data would create a machine with multiple accounts holding tangled permissions in all directions.
And then, yeah, the end of the story is I discovered Network Recovery in this thread, erased the hard drive all over again, and basically installed Mountain Lion on the empty machine using my slightly older Time Capsule from a much healthier external drive.
Very NOT-intuitive, doing an OS downgrade that way, I would say, but it worked. !Yay!
I am not completely positive Network Recovery is necessary here? Does the suite of Recovery software stored on the hard drive partition offer the option to restore from a Time Capsule ---- OR, since Network Recovery is designed for machines having a hardware issue with the volume originally hosting the OS, does only Network Recovery offer that option.
Anyhow, thanks again macrumors, and someone less verbose than I can probably write up some of this technique much, much better.
