External SSD will not restart, reverts back to internal HD
I have cloned my internal HD to external SSD connected thru USB3 port, and it starts and runs fine using Startup Manager (hold Option key). However, whether selecting the external drive from Startup Manager (using Control key), or from Startup Disk in Sys Prefs — it reverts to the internal HD on restart. No firmware password, No T2 chip. Late-2013 iMac, so only able to run Catalina - but, it is up to date. How do I get it to ‘stick’ on the external startup disk?
iMac 27″, macOS 10.15