2020 Intel iMac - Big Sur won't install after format

[Tsubame]

Currently using a late-2020 intel iMac 27 inch. Has worked like a dream, but ran into a weird issue the other day. Was booting back to OSX from my Windows 10 Bootcamp partition, and getting the error that it couldn't boot into Macintosh HD because the file system wasn't found. Decided to try reinstalling the OS to see if this would resolve the situation, but it couldn't install Big Sur.

No problem, most of my files are in the cloud so I did a full erase of the drive via Recovery boot and tried reinstalling Big Sur again. Still no go. Strange, but I booted into the base iMac Recovery (command,ctrl,shift-R) and installed Catalina again, figuring I would upgrade to Big Sur after.

Good News: Catalina installed just fine and everything is working just fine.
Bad News: I STILL can't get Big Sur to install. It downloads the installer without a problem, but when I run the install after about ten seconds I get "An error occured loading the update". Online research showed this usually is a disk space issue, but I have close to 1TB free since I fully wiped the drive.

Any ideas? I have seen a lot of various permutations of this issue online, and while I could just use Catalina it is pretty frustrating.

Incidentally, this all happened the day before my HD to set up a TimeMachine backup came in. I have it now so if I can get this situation resolved hopefully I can prevent it ever happening.

 

[cqexbesd]

As no one else has replied I will give you some wild guesses.

1. Use disk utility to check your drive is correctly formatted (APFS I guess?) and there are no odd partitions etc. I don't expect this is the problem.

2. Use dtruss or similar to see what fails in the installer.

Not sure that will help you much but who knows...

 

[Tsubame]

Thanks for the reply, appreciate it.

1. Checked the formatting and partitions of the drive, everything seems totally fine. Removed the Bootcamp partition as well before starting all of this. I figure that might be somehow related because bootcamp is really the only thing I have done that would mess with the drive partitions. Still, it was working fine for months, regularly switching OS, before this problem cropped up.

2. Will try this one next when I get back from work, thanks for the suggestion. Figuring out what the error is besides the imminently useful "An error has occurred" would go a long way.

 

[Tsubame]

Quick update if anyone was interested.

Very randomly I just tried to give the Big Sur install another shot after work today, and the whole thing went through. Now I wish it didn't.

I am experiencing some pretty insignificant stability issues. It seems to work absolutely fine for a while (even handling graphical intensive tasks like games just as well as it did before) and then just randomly hard crashes. Sometimes it is just the app running (FFXIV at that time), I have had it suddenly log me out of my account, and a couple times it actually fully restarted the machine with the kernel "your mac has restarted unexpectedly" error. I got about 7 crashes in the first hour. Nothing is actually throwing any errors, it just constantly breaks.


I may need to wipe the whole drive again and just go back to Catalina and stay there, because this is getting kind of silly.