Problem using external SSD as Mac OS Monterey Start-up Disk (iMac late 2015, 27" Retina, 17.1)

Hello,

A while back my internal SSD failed and I resorted to using an external drive (Samsung, 1TB, SSD, ca. 2015) as start-up disk / regular main drive for my old iMac. I am using OS Monterey.

Problem is that as soon as there is the shortest power-off (be it a regular system restart, shut-down, or a power outage), my iMac will restart normally, but after a few minutes I see the message "the disc you attached was not readable by this computer" (Disk Utility repair does not fix the problem). After that I have less than 5 minutes before my iMac shuts down due to an unexplained error. The iMac will start again and the loop of unexpected error -> restart - unexpected error continues indefinitely.

I already tried creating another bootable USB start-up for Monterey and clean install. I also tried using multiple other external drives, but nothing solves the issue.

I am running out of ideas and would appreciate any help.

Thanks,

Oliver