System does not start after update(( iMac 2109

I wouldn't use migration assistant or time machine.

Once you install macOS on the external drive and boot Mac from it, you'll be able to see the internal drive of your Mac.

You should then copy/paste your data manually.

Yeah, hard work, but it will make you sure of copying everything you need, you get better control over the process, you can do that by steps, checking if the most important files have been copied accordingly and they're not corrupted.

I know Migration and TM are here specifically for that job, but believe me, a ton of people has problems when using them, specially when a lot of data is involved. And when the do it the hard way, they always end up happy and satisfied. You'll not regret.

Please, be very careful when formatting the drive to install macOS so you don't erase your internal drive.

Get an external hard disk and run the recover mode it after formatting the external hard disk for Mac OS X. Make sure it has sufficient space to migrate your data and give you an opportunity to clean up unnecessary files. Once done, then use http://www.bombich.com/ Carbon Copy Cloner to clone back.

Note, you should never let your system get over 85% full. The fact you were left with 150 MB of data, says you were nearing capacity before the last update started.

Always keep a backup of your data before installing anything new.

Most likely any 1 TB or 2 TB external hard disk will do. They are $60-$70 at Staples.

If it is a 2019 iMac, you have both USB and USB 3 ports to connect the drive to.

Once connected, in recovery mode, open Disk Utility to locate the hard disk. It should have its manufacturer name. Erase and format it HFS+ Extended Journalled.

Once you have run the installer and install onto the external hard disk.

It will then upon completion ask you to use the Migration assistant to migrate data, which you do. That will bring over all the data on your internal drive.

As long as your Mac's hard disk is smaller than the external disk in capacity it should work.

Note 500 GB is less than 1 TB.

oh, before all that, we can try to do something for that installation you already have

First is to get to Recovery Mode and open Disk Utility. The click View button and select Show all devices. Select your main drive and run First Aid. If you see any errors, run it a few more times.

Also try booting in Safe Mode by starting your Mac holding Shift. That boot wakes a while because macOS gives himself some maintenance. That can solve a lot of problems. Maybe it does that for you.

You'll love the result.

Anyway, after a problem like this, I'd say you should execute the first plan. Create a bootable pendrive, install macOS on external drive, copy your data, wipe your internal drive, clean install macOS, copy back your data from external, and you're done. Fresh and clean macOS with no corrupted files and settings.

Hi, can you, please, post here some pictures of finder left menu and Disk utility (with View set to Show all devices)?

Note, this tip explains how to format the external hard disk:

https://discussions.apple.com/docs/DOC-3003

Great! 😃

Nice to know it worked and now you have your 💻 up and running smoothly again.