iMac 17,1 has no system on reboot

Hi everyone,

I have a "late 2015" iMac 27" Retina running macOS 12.7.5 from a USB SSD. The internal drive is not formatted and not in use.

When I freshly start the Mac after a power off, everything works fine. However, if I restart it ("Restart..." from the Apple Menu), it doesn't find a system and only shows the Question Mark screen. As soon as I turn it off and then on again, it boots without problems.

So, in summary, I can start the Mac, but can not restart it.

The external USB SSD is APFS formatted. MacOS 12.7.5 is the latest system running on this iMac. The external USB SSD ist set as Start Volume. I don'T want to use the internal drive, since it's a very slow HDD and can't be swapped easily.

Do you have an idea what causes this behaviour, and how to fix it?

best,

geb