Metadata corrupted after Sequoia update

What does Disk First Aid say when you run it in Recovery mode from bottom to top in that list at the left (choose View > Show All Devices first)? What name should the internal boot volume have? Make a backup before trying any fix.

Use Disk Utility to rename the top most item shown in your screenshot where it shows "+ volumes". Theoretically it should automatically rename the next two items below it as well. If not, then rename those other two items currently named " + " to the same name you gave to the first item.

patrickt_appleid wrote:

But what should I rename it to? Is there a conventional name for it?

"Macintosh HD" is the default from the factory, but you can give it any name you wish as long as it is unique from any other connected drives/volumes.

I'm too scared to run First Aid now. But when I did run it, it said something along the lines that 'no problems were detected' and gave me green flags all the way (I ran the First Aid on all the volumes shown in the screenshot above).

The First Aid summary can lie (personal experience). Make sure to click "Show Details" and scroll back through the report to see if any unfixed errors are listed (even some warnings are a problem, but not all). If there are errors, then run First Aid again until the errors are gone. If after several attempts the errors remain, then you will need to run First Aid while booted into Recovery Mode. Make sure to run First Aid on the Container as it will also automatically scan all the APFS volumes within that Container.

P.S. I notice that I have filled my hard drive with 447GB/494GB. I'm not sure how that's possible as I usually keep it below 300GB of used space.

APFS backup snapshots are one place where data can be hidden even after it has been deleted.

If you manually backup your iPhone & iPad backups, it can surprise people by how much storage they can take up since they are hidden away within the hidden Library folder within your home user folder.

Maybe a cloud file syncing service is bringing down items that are filling the drive.

You can use a utility like OmniDiskSweeper to show you the largest files/folder to help you locate where those large items may be located. Please don't use any cleaning/optimizer apps because they usually cause more problems than they solve plus the interfere with the normal operation of macOS.

Also, ignore the "Available" storage value Apple puts everywhere since it is very misleading. The Free space value shown in Disk Utility is the one you want to use to monitor the storage space.

patrickt_appleid wrote:

I'll do the renaming after a complete back up of my hard-drive (including invisible files and such), so that may take me a couple of days. But what should I rename it to? Is there a conventional name for it?

It is always good to make a backup before any major (or minor) updates or fix attempts. Better yet, make two different backups -- one with Time Machine and another with Carbon Copy Cloner (SuperDuper is another option but I have not used it). A belt and suspenders. Very rarely there have been reports here that restoring from a Time Machine backup did not work for some obscure reason.

It is also good to have another bootable volume, just in case. For example, a bootable macOS installer or a minimal external boot volume (verify that it really boots. Internet Recovery is another option but it is slow and might sometimes be flaky):

Create a bootable installer for macOS - Apple Support

p.s. I have an old habit to name the main internal boot volume just as "HD" so I don't have to escape spaces in the Terminal. Test volumes on external disks are named as "macOS_15_test" etc.