Sorry for the confusion. It was the discussion of struggling to revive a Fusion drive that caused me to post my question.
I think I'll just run whatever comes with the iMac on the internal 256 Gb and use an old 256 Gb SSD I've got in a caddy connected by USB-C or USB-C 3.1. I presume I'll be able to boot from such an external drive?
Any suggestions?
iMac (19,1, 3.1 GHz i5, 12.7.4, 40 Gb RAM); MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) Vodafone router and Devolo Wi-Fi Extender, Canon TS8351 printer/scanner.
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