Restarting iMac from an external drive

I recently acquired a 1Gb Samsung external SSI drive, which I formatted with Monterey OS. In order to boot up from the Samsung, I would restart the computer while pressing the option (alt) key, which worked fine. For some reason, System Preferences>Startup Disk did not show the Samsung drive, and therefore rebooting from there was impossible. Recently, however, restarting the computer under Monterey by pressing the option key has started failing: sometimes I have to make a couple of attempts before it boots. Now it can take as many as six to eight attempts. What can be causing this, and what can I do about it? Thanks in advance!

(Just to be clear: what happens when I try to restart is first a black screen, then the Apple logo and a chime (sometimes sharper, sometimes flatter), and then I land on the High Sierra login screen).