- Disconnect and store the bad HDD for the time being
- Purchase and install a new SDD (preferably SDD, not HDD) of the appropriate capacity
- If your HDD contains far more data than will fit on even the highest capacity SSD, then of course purchase a new replacement HDD
- Install and format the new SSD (or HDD) as APFS
- Boot into Recovery and select the option to restore from Time Machine Backup
- Follow on-screen instructions to restore everything to the new SSD (or HDD)
I have about 3T of data. Not sure what went wrong with the HDD, but I do have an enclosure I can slap it in and hope for the best.
I already have a new 6T HDD. My existing SSD (with Sonoma but not user folder on it) is 500g, which is potentially plenty for my system, OS, apps, and user folder so long as I keep all the photos, videos, and music on a separate drive.
I booted into recovery, formatted the 6T HDD as APFS, and attempted to restore from Time Machine and got the following error: "
You must use Migration Assistant to transfer data from this backup."
How?
UPDATE: I can boot from the broken HDD. I get an error. Paraphrase: "Disk can't be repaired. Backup what you can. You can read but not write". I tried opening
System Preferences --> Users & Groups with the goal of migrating the User Folder back to the SSD, but it's the one System Preference I can't seem to open at all.
UPDATE2: I logged into the second user account and gave it admin, then restarted and logged in again. I opened
System Preferences --> Users & Groups. By right clicking on my main user account, I managed to change the user folder to the SSD. Then I rebooted successfully into my user account and ran Migration Assistant, selecting restore from Time machine backup. It's complicated since Time Machine only let me restore the system automatically. Most of my data I had to do manually. Time Machine is a crappy backup by itself. I learned my lesson. No more splitting up the User folder and OS. Definitely dedicating a separate external drive for CCC backups.
SOLVED
UPDATE3: Predictably, since formatting my data HDD to APFS and moving my User folder to my SSD, my system is MUCH snappier.