Sierra won't switch between OSs.

Running Sierra on a late 2013 27" iMac. Have 2 separate partitioned EHDs handling my Music, Photos and Movies. I want to try Catalina OS, so have a Crucial X8 SSD EHD, installed Catalina OS on it, and it was fine for awhile. Was able to set up the basics for the OS, but did not transfer my Sierra "stuff" yet.

Switched the startup OSs via System Preferences > Startup Disk a few times, just for practice. Then suddenly the switching function stopped working. Although Startup Disk confirms the Catalina OS as the restart disk, Sierra comes up on restarting. I can't bring up Catalina although it remains a choice within System Preferences > Startup Disk.

On the sidebar of Finder, Devices lists Catalina OS and Catalina OD - Data, as well as my other EHDs.

What is the cause of my problem, and how can I correct it?

Thank you.