Can I delete SystemMigration folder, including the history?

FYI, I just tested a Mac in Recovery Mode and the "visudo" command is not available in Recovery Mode which makes fixing the problem properly much more difficult. I recommend grabbing a good copy from a backup instead, or maybe copying a default file from another Mac (assuming that file has not been modified).

Otherwise performing a clean install of macOS by first erasing the disk followed by reinstalling macOS. When restoring from a backup....make sure to avoid migrating any system settings (uncheck that item if restoring from a Time Machine backup).

EllenBEX wrote:

Thank you for the sudo detail! Regrettably it is 90% Greek to me. I don't even know what a sudoers file is or how to change it. As I said, this change happened when I tried to update from Mojave to Catalina, and the update failed, and thrashed my computer. I had to do a complete restore from Time Machine, back to Mojave, and it mostly worked.

The OS upgrade is unlikely to have caused this particular issue with the "sudoers" file. It may just have exposed the latent issue.

Hopefully it will self fix when I migrate to a new computer.

If you migrate or restore the main system settings (if using Time Machine), then you are likely to bring the problem back over. If you uncheck "system settings", then that theoretically should prevent that file damage being brought to a new system. However, since that file is damaged, who knows what else in those system configuration files could have been improperly altered.

Thank you for the information, and I now know what to do to fix it. The computer is 10 years old - may need a new one soon.

If you have at least 80GB+ of Free storage space on your internal boot drive, then you could create a new APFS volume (give it a unique name) and install macOS onto that new volume so you can get access to a new pristine default copy of the "sudoers" file. You can also install macOS to an external USB3 SSD (even a Hard Drive would work although it will be extremely slow for installation & booting, but it is only just to fix the broken OS. You can then copy that file to the broken OS while booted from this clean OS. When you next boot into the broken macOS theoretically the "sudoers" file issues should be resolved. It is a bit tricky locating the proper location within the broken OS, but I can provide the information if you want to try this fix.

FYI, ignore the "Available" storage value shown everywhere within macOS since "Available" is not synonymous with Free. Use Disk Utility to check the actual Free storage space of the Data volume.

Also, there is no need to restore anything from Time Machine when configuring this second macOS installation as nothing else is needed than a clean OS just long enough to apply this fix. Afterwards you can delete the newly created APFS volume or erase the external drive depending on which option you chose.

Those errors were caused by improper changes to the "sudoers" file since the permissions on that file are incorrect as mentioned in the error message itself, plus there that file is reporting there are no "sudoers" users assigned or configured. It has nothing to do with any old user except perhaps that user account incorrectly edited or modified that "sudoers" file.

I don't think there is any easy way to fix this problem if you cannot use "sudo" since "sudo" is required to fix this problem. Perhaps it can be fixed while booted into Recovery Mode, but it will be that much more difficult because you must be able to point to the proper location of that file instead of using the defaults.....assuming that the "visudo" command is even available while booted into Recovery Mode.

I guess you can try restoring that "sudoers" file from a backup made prior to that file being damaged (would require a reboot afterwards).

EllenBEX wrote:

((why doesn't Apple keep me logged in? Such a nuisance))

Security.

I don't know if this drive can be partitioned without wiping.

I did not suggest partitioning. I suggested creating a new APFS volume which is not a partition, although it acts a lot like a partition, but without needing to resize anything. Creating a new APFS volume allows that volume to sit within the same APFS Container as your current OS & sharing the same storage pool. This is why I suggested this option since there is very little risk. Of course you can use an external USB3 drive instead, but that may incur a purchase cost if you don't have one which you can erase.

In any even, if I migrate it will be to a new computer with a new OS in place, and I will be SURE not to migrate the system settings.

And, it was absolutely the update that killed the system. It worked before the update, but not after the update. Maybe it was Time Machine restore made necessary by the failed update that made the changes, but I don't see how. Trust me: no one messed with the sudo files. No one here knows how to mess with those files.

I don't see that "sudoers" file becoming broken with having no "sudoer" users and also having permissions issues. Someone or something (perhaps a bad app you installed) incorrectly modified the "sudoers" file. I don't see a macOS update doing this type of damage (two types of damage), but I can see this damage may have gone unnoticed prior to the OS update/upgrade. How often do you use the command line and "sudo"? Does anyone else have access to your Mac and knows a password for a macOS admin user account?

It would be interesting to see the contents of that file. I wonder if there are any other bad modifications within it. Personally I would be a bit concerned especially if you have no clue about this file.

You can delete it, but if you have migrated it can be that you'll need to install macOS anew afterwards