Can't startup from external USB SSD
After recent system security update, late 2013 imac 27" running 10.14.6 won't recognize external USB startup SSD drive. I've tried setting the drive as startup drive and restarting holding the option key down but the SSD is not recognized. Once the mac starts up from the internal HD, the SSD appears on the desktop. This mac has been running from the external SSD for a long time prior to the most recent update. Drive seems to function properly, but imac refuses to recognize it on start up. External SSD drive is formatted APFS. Internal HD is GUID. I can't update the OS any further because this computer must run several older applications that won't work in 10.15.
I have tried 3 different SSDs - same problem for all. The other 2013 27" imac we have also reverted to the internal HD after the security update, but I was able to get it to recognize the SSD with option key held down on reboot then reset the SSD as startup drive.
I'm at wits end and have wasted 2 days fighting this problem after installing what appeared to be a harmless update. Any help is greatly appreciated.
iMac Line (2012 and Later)