In case anybody was wondering where this currently is, it turns out that the original hard drive from the old iMac has some kind of mechanical damage, and was not a matter of being simply erased. This comes as a surprise, as I was encountering no problems with the computer up to this point.

Unfortunately, this precludes any kind of simple software recovery. Meaning the next step is a "clean room" recovery, with a technician in a bunny suit picking manually through the data. Cost runs from a couple to several thousand dollars.

But wait, what about the external backup drive I was using? Though a little overwritten, most if not all of the original data should still be intact, yes? Turns out that yes, the files were recoverable and I now have them in my possession. Cost about $500.

It still means I have to pick through thousands of files one-at-a-time. There are so many files on the recovery drive (many duplicates due to how Time Machine works) that it took several hours just to mount the hard drive to my Mac Mini. But at least it's all there.

You know what they say: “two is one and one is none". It’s a good reminder for me to get on the ball and set up an off-site backup.

^Exactamundo. In this aftermath, I renewed my Carbonite subscription.

My backup strategy:

Two LaCie portable USB disk drives. On odd days of the month I do a Time Machine backup to one. On even days of the month I do a Time Machine backup to the other. If one disk fails the other will have a recent backup.

How I do my backups (auto backup is disabled). Once a day I:
* Disconnect from the Internet.
* Command a Malwarebytes scan.
* When the scan completes w/o errors I command a Time Machine backup.
* When the backup completes I reconnect to the Internet.
That way I can be reasonably confident that all of my backups are clean.

Since I backup only once a day there’s a lot of data to backup. So I disabled the Mac’s backup throttling mechanism so that the once-a-day backup takes less time.

I like a program called SuperDuper for Mac. One for one, bit for bit duplicate HD of the main onto an external. It's actually bootable, and browsable. I alternate from red to blue (they sell drives in color (Samsung). I do that weekly.

I also use TimeMachine to a network connected NAS drive.