iMac won’t boot from external SSD I’ve been booting our 2013 iMac from an external USB SSD; the internal Fusion drive is flaky and I’ve associated it with repeated crashes. But after installing Catalina Security Update 2020-001, the external SSD is no longer recognized at boot time. If I restart while holding down the Opt key, the startup manager shows only the internal Fusion drive. I can still boot from the Fusion drive (though it’s not reliable for regular use), and when I do, the external SSD is visible and can be mounted. I’ve tried loading additional drivers from the startup manager (Opt-Shift-Cmd-Period) and resetting NVRAM (Cmd-Opt-P-R), but I still can’t boot from the external SSD.  It was working fine until yesterday.  Can anyone suggest another solution? Thanks in advance!
I mentioned in the original post that I had reset NVRAM (twice, as it happens). I reset NVRAM a third time, and lo and behold, could now boot from the external SSD. Go figure.