Some Uno and Nano clones not recognized (by computer. Leds light up)

Hi,

I have a few boards that are not recognized as usb devices and I am trying to find out if it is a soft or hard problem.

M.Jduino uno
and a
nano clone. Same behavior:

PWR lights up steady, L flickers

On three different Macs with OS High Sierra or Catalina the Arduinos does not show up even in the OSX system report (about this mac...). Naturally they do not show up in the Arduino editor either.

I have tried many different USB cables, including cables that work with other Arduinos.

I have other unos and nanos that work fine (after getting the right drivers and so on).

Should I consider these lost or can there be a way to make them work?

Thank you for your help!

Please let me know if I should provide more information.