Booting iMac from external USB SSD no longer possible

I have an older iMac (2011) and am currently running Catalina, which is the last upgrade for that model. I have always been able to boot from my external USB-connected Samsung SSD until the last few months. If I choose the SSD as the startup disk in System Preferences and do a Restart, it reboots from the internal HDD instead. After the system is up and running, it will recognize the external SSD and can access it, but it is still running off of the slow internal HDD. The firmware option to allow booting from external drive is no longer available in MacOS Recovery. The only option on the System Security on Utilities is to set or remove a firmware password. Are there others in this community that have lost the ability to boot from external drives and have found a workaround?