OS became corrupted

artstype wrote:

I am attempting to backup with Time Machine. I have always used CCC for backups before, so I am not certain that it is backing up the internal drive (as I obviously have booted from an external drive). From what I’ve read, one must specifically exclude things, as the default is to backup all volumes. It would be great to know if I understand this correctly. In the meantime, I will also clone the drive when TM is done.

I've never used TM so I don't know the particulars.

Instead of trying to install Big Sur use the Big Sur installer to run Disk Utility First Aid on the SSD. In fact I recommend running First Aid on the Container. To select the Container you must first click on "View" within Disk Utility and select "Show All Devices" so that the physical drive and Container appear on the left pane of Disk Utility. Even if First Aid shows everything is "Ok" click on "Show Details" to see if there are any unfixed errors. If there are unfixed errors this will require erasing the drive and restoring from a backup or clone.

If First Aid is unable to repair the volume, then install macOS from Internet Recovery Mode to an external USB drive. Even if this installs macOS 10.12 to the external drive you should then be able to access the files on the APFS volume since 10.12.6 can mount APFS volumes although you need to access the Catalina "Users" folder from the "Macintosh HD - Data" volume where the actual Catalina user accounts are stored. If you have trouble with maOS 10.12 accessing the data, then at least you can then upgrade the external drive to Catalina. I would not attempt to install Big Sur until after you resolve these issues. I personally spent two days trying to install Big Sur (the failures only had cryptic error messages) and I've seen lots of other posts where users have had Big Sur issues. You may just be compounding the problem by upgrading to Big Sur.

Make sure to always have frequent & regular backups. FYI, it is impossible to recover accidentally deleted data from an SSD plus an SSD can fail at any time without any warning signs.

See if your laptop qualifies for this free Apple SSD repair program:

https://support.apple.com/13-inch-macbook-pro-solid-state-drive-service

FYI, here is the current list of the publicly acknowledged free repair programs for various Apple products (some products such as your laptop have multiple programs available):

https://support.apple.com/service-programs

Time machine is smart enough not to try to back up the backup drive, if that is what you are asking.

You can check its 'excluded' list in time machine preferences by clicking the (Options) button at the lower right to get the "exclude" list of drives, directories, and files you would like to exclude.

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You will need to backup or clone the SSD, then erase the drive so you create a new clean file system. Then restore from the backup or clone.

Disk Warrior is a great app for repairing the HFS+ file system, but unfortunately Apple has not released the necessary APFS documentation so that third party utilities can repair an APFS file system. It is a shame that Apple cannot design First Aid so that it can actually fix file system issues.