iMac not booting from external bootable drive

Hello, boricua61.

Shut down the computer and disconnect any devices.

Then give these a try. Do each one three of four time in a row - sometimes that's what it takes.

PRAM reset:

How to Reset NVRAM / PRAM on your Mac - Apple Support

SMC reset:

How to Reset the SMC of your Mac - Apple Support

Then plug in your devices and restart your Mac.

You will want to visit System Preferences and confirm the startup drive again.

If this doesn't work, you might also try a safe mode restart.

How to use safe mode on your Mac - Apple Support 

Does the problem persist while in Safe mode? 

Exit safe mode by restarting your Mac normally and re-evaluate the issue again after a normal reboot

Try booting holding down option key and see if both drives appear. Then pick the one you want. See if now it remembers the correct one.