OS not starting up, prohibitory symbol

Question

I have a Mid-2012 MacBook Pro with Boot Camp, and it supports only up to macOS Catalina, and not Big Sur. This frustrated me and I decided to go off the rules a bit & installed macOS Big Sur (using patched Sur). This was all working fine, until a few days ago, I reseted the PRAM as I was having some issues with the CD-ROM.

The problem started here. After I reseted the PRAM, the MacBook started showing the prohibitory symbol. It scared me a lot since it had a lot of data (class notes, reference books for exams, etc.), and for the past 3 days, I've been searching the internet for its fix. So, by now I've come to know that this sign comes when the Hard-Drive contains a macOS, but it's not supported on the Mac. Obviously, it contained Big Sur, which was not supported by this model.

On doing Internet recovery, it gave an option of reinstalling Mac OS X Lion, but when selecting a disk to reinstall the OS, it shows only the Boot Camp partition of the Hard Drive and shows the message that it's locked.

So, I wanted to ask if there is any method of reinstalling macOS (and obviously this time I'm not going back to Big Sur, only up to Catalina. This experience was enough) without losing any of my data, both from Boot Camp and macOS? If not, then is there a way I could back up files of macOS partition from Boot Camp partition? Also, is there a simpler way to bypass this check, like the bypass created by Patched Sur, as that'll mean I'll not have to go back to Lion and reinstall OS updates to go back to Catalina?

Also, I wanted to know what exactly happened when I reseted the PRAM, that this bypass stopped working? This needn't include in the answers, but info on how this happened will help in expanding my knowledge.

Answer 1

Firstly, you should always have a backup of your files, so there should be no question of losing anything. Files without a backup are waiting to be lost.

If the Recovery partition is indeed on Lion, then it won't read the new APFS formatted disk that Big Sur uses.

Your first port of call should be to whoever wrote the patch, and ask them about whether it alters NVRAM in any way. It's possible that the reset is unrelated, and merely coincidental, of course.

I'd suggest that your best course of action is to get an external drive that you can install some working OS to - even if it's Lion, and then upgrade that to Catalina, which at the least will be able to read the disk. Even a cheap 64GB 'thumb' drive may be sufficient in extremis, (though obviously, you'll be buying an external for backing up anyway...)

You should be able to install Catalina on the internal drive once you're running Catalina on the external.

Contrary to popular opinion, Apple don't cut off hardware from new OSes on a whim -- it's usually because the hardware lacks some specific requirement, or the 'experience' is brick slow.

TBH, there's not much that you'll gain from Big Sur: Catalina will run perfectly well and should be compatible with new apps for a few years yet. But ultimately -- it's nearly a 10 year-old laptop. It may well keep running fine as it is for years, but it won't keep pace with the shiniest anymore.