Looking for the best backup option under circumstances
Solution 1:
After reviewing a few suggestions, and after some other minor issues (the workspace Catalina partition I had created stopped booting, so I removed it and replaced it with a High Sierra installation), I opted for the following:
- Formatted the SSD to HFS+ (since I was using High Sierra)
- Downloaded Carbon Copy Cloner through the workspace partition. (Disk Drill, mentioned above, cost more than the drive, while CCC offers a 30 day trial.)
- Cloned the original partition to the SSD (took about 4 hours, no errors found).
- True to advertising, I WAS ABLE TO BOOT THE SSD.
I haven't browsed every file; I imagine there may be a few lost, but this is still leaps and bounds over reformatting and losing it all. CCC apparently turned the SSD into APFS format, so I went ahead and updated to Catalina by downloading the installer.
As an aside, I did try originally using DiskUtility's "Restore" function to copy the partition, but this failed with errors.
As noted above, the partition had become unreadable due to a software issue. I had manually rebuilt the partition map so felt confident I wasn't going to affect file tables by working in the space outside of the target partition. If this had been a mechanical issue, I would have been far more concerned about using the drive as a workbench which, while not impossible, would have made things more difficult.