ME Mini here and I have the same issue. I have populated it with 4 4TB drives configured in Raid and one 1TB drive for OS. I have ZimaOS set up and it boots and runs great. However it just goes unavailable and I have to reboot. The uptime can vary from a few hours to about 1 day so far. The only thing running at the moment is Home Assistant, Immich and Glances and I’ve tried to identify CPU spikes. I’m hoping somebody can tell me how to diagnose this behaviour.
With a monitor plugged in, I can see a command line scrolling through errors but no way to stop it or interact with the machine unless I do a manual reboot. 

Thanks, Daniel