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I mentioned a while back that my 2012 iMac’s hard drive finally died, which led to some tricky choices: to replace it altogether, or replace the hard drive with an SSD, or with another hard drive (not recommended nowadays), or a fusion drive where the system treats a dual SSD and HD as a single drive, or—my eventual choice—a 480GB SSD and a 4TB hard drive installed as separate drives, with system and everyday files on the former and archival files on the latter. That way I got the full speed boost of running my system and apps on an SSD, with the space for archives of a honking great hard-disk. I got some extra memory installed while I was at it, so that the revamped Mac would last me at least a few more years and save having to spend two grand on a replacement.

Everything went fine, and I had a recent enough backup that I lost no files worth worrying about, although there were the usual software updates after the transition to macOS Mojave. But in copying stuff over from my backup, I made a couple of mistakes that led to some month-long headaches. This post serves as warning and solution, if you’re ever in the same boat.