macOS 10.15.7 update will not let me boot from my external startup drive.

I have been using a Samsung T5 external SSD successfully for a few months now and what a difference it has made to the speed of my late 2013 27” iMac. From hitting return on my user password it takes 3, yes 3 seconds to finish booting.

Since the 10.15.7 update a few days ago it will not boot from the external drive. I can view the external drive in finder, I can select it as a startup disk in system preferences, System Preferences tells me I have selected it but it boots from the internal hard drive. And it’s so slow!

When I hold the Option key down at startup the external drive is not listed. Only the Macintosh HD. It’s like the startup sequence waits for a while, can’t find something on the external drive, the says oh well I’ll use the hard drive to startup.

Any ideas?