Attempting an erase and clean install of Big Sur on iMac, encountered (potential) issues after erase.

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Attempting an erase and clean install of Big Sur on iMac, encountered (potential) issues after erase.

iMac 2019. Was previously running the most up-to-date Catalina. Did an erase per the instructions here : https://shttps://support.apple.com/en-us/HT208496 (Disk Utility had APFS selected automatically, so I went with that, per the support article). Then, from the recovery menu, did an OS re-install. Except at that point it installed Big Sur (11.0.1). I didn't expect this, I just selected OS installation from the recovery menu and this is what it gave me, but I didn't think it would be any problem and I didn't perceive any issues with the OS.

After that, due to some issues caused by screw ups with setting up Apple IDs and accounts, I decided to erase and start over again clean. I followed the same instructions, but this time, after the erase, I looked at disk utility in recovery again to see what was left afterwards (because I'm just now learning how very different APFS is).

Things that stood out to me:

Apparently, erasing and reformatting took me back to a macOS Base System Catalina (10.15.7).

According to Disk utility the 1 TB of space is shared by 5 volumes. 3 of which are not mounted and take up 7.74 GB. I am not sure what those 3 are, even after running diskutil list.

When I ran diskutil list in terminal it listed 24 disks/disk images. Twenty-four. Aside from disk0 - disk3 – which have identifying info that makes sense (for the most part) – there are an additional 20 "untitled" mystery disk images. They're all of varying small sizes, from 524.3 KB to 12.6 MB.

In Terminal, under disk3 (synthesized), I see an APFS Volume named "VM" (Size: 3.2 GB, Identifier: disk3s5). What is that? Virtual Machine? Virtual Memory?

And why does disk3 say "synthesized"? Because I erased the HD and this is a sort of placeholder?

I am wondering if it is even a good idea to attempt an OS reinstall at this point. The 20 extra untitled "disks/disk images" seems wrong, to me.

If I should reinstall, do I need to start with Catalina instead of Big Sur? Since that is the OS now indicated as the macOS Base System?

(Prior to the erase I had downloaded Big Sur and made a bootable installer, just in case, FWIW.)

Any good advice would be massively appreciated.

iMac 21.5″, macOS 11.0

Posted on Dec 9, 2020 9:07 PM

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Answer 1

auroranm Author

Level 1

4 points

Dec 10, 2020 1:18 AM in response to auroranm

After some digging I found some answers to 2 of my questions. "VM" is Virtual Memory, and disk3 showing as "synthesized" is normal (I still don't know what it means, but it's normal).

I'm still concerned about the extra (?) 20 untitled disk images.

Also, I do understand that it's normal (in APFS) to see "other volumes" sharing space, and those should be Preboot, Recovery, and VM. What's confusing to me, however, is that those (combined) don't come close to the 7.74 GB that Disk Utility says they are using.

I ran First Aid on everything. All came back "ok" but there were a couple of warnings:

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