Okay, Apple are really starting to freak me out (I don’t use them much). Here’s the timeline of my story:
My daughter’s MacBook Air (2017-ish) that she needs for school would not update to Big Sur because it had a pityingly small 128G SSD. So, I decided to spring for a 500G one and get her device to a more acceptable level.
I first tried this on my less important Mac Mini, swapping out the 500G for a 1T, though still spindle rather than SSD (I don’t need the speed on that box). A Time Machine full backup was done in preparation for restoring, the new drive was inserted, and I used recovery to get everything back. That worked well.
Daughter’s machine, not so well. Trying to do a normal recovery with command-R and restoring from Time Machine, resulted in a failure, the text which I don’t remember. But it did seem very fast compared to the following step.
Doing it with command-shift-R seemed to work but consistently ended up rebooting with a dreaded:
So, I thought, forget the automatic recovery, I’ll just install Big Sur and try to get the data from Time Machine afterwards (I’d read a migration would allow this). I left the install running and went to work.
The first thing I noticed was that it didn’t ask me for my wi-fi password, despite the fact it was a brand new SSD. I can only assume certain stuff is stored in the Mac somewhere, separate from the drive.
The thing that really amazed me was that, when I got home, the machine seemed to be fully done. It had my daughter’s account information, all her files that she knew about, all the apps she had installed, the screen-time settings, the school stuff like Office 365, and everything that I know to look for.
This is despite the fact neither the Time Machine backup nor original SSD were available during the Big Sur install. I know Mac aficionados say it "just works" but this is bordering on "horror movie" scary.
I guess what I’m trying to ask is:
How did this actually manage to do all this? Is there more stuff than just the wi-fi password stored on the device outside of the disk? This is about the only way I can see this happening, though it couldn’t possibly be a large amount of data, surely?
Can I be reasonably certain it did it properly? I want to avoid the drama of visiting IT support next year when she starts school, because I may have stuffed something up.
Though iCloud does store more than just Wi-Fi passwords by default (assuming you're using it), it isn't the primary culprit. Unlike some other operating systems, macOS doesn't wipe everything when installing (or reinstalling) the OS by default. While I don't have a definitive list of files it touches (and it likely varies by macOS version), you can typically assume that it will primarily touch system-related files.
What appears to have happened in your case is that your initial "failed" installation actually copied over many of your personal files successfully from the Time Machine backup; however, for one reason or another, something related to the OS installation failed and prevented the drive from being made into a bootable disk. When you re-installed macOS after the fact, those files remained in place, and macOS simply installed the OS alongside them.
Given the fact that the initial Time Machine migration failed, the exact state of what was restored isn't entirely clear. I'd recommend wiping the entire disk via Recovery Mode, reinstalling, and then using the setup utility to restore from Time Machine. Remember to format the SSD as APFS rather than HFS!
Correct answer by JMY1000 on December 12, 2020
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