It seems that I haven't configured the external drive properly before choosing it as a startup. Therefore, upon restarting, the iMac showed a circle with electric flash roaming around the screen
iMac 21.5″, macOS 10.15
Community user @Jack19 authored this User Tip for using an external drive as a startup drive on Mac. It says SSD, but the steps also apply for using an external HDD as well.
Use an external SSD as your startup disk … - Apple Community
Have you unintentionally started the Mac in Target Disk mode by holding down the T key? Power off the computer by pressing the power button until the computer shuts down.
If you are trying to restart from a properly configured external drive, one correct process is to press and hold the Option key when you power up the computer.
Do you get one of these screens: If your Mac doesn't start up all the way - Apple Support?
Or are you referring to this screen (Target Disk Mode):
What version of macOS is on each drive - internal, external. How is the external connected?
Exactly what model Mac do you have ... while your profile says iMac 21.5" that isn't specific enough.
Restart after choosing an external hard drive as startup
