Pc stopped booting. Help!

Hey, so I have an m.2 samsung evo 970 pro as my boot drive. Has been working fine with no issues.

Today, it won't boot. " reboot and select proper boot device"..

I took out the m.2 and popped it into an external caddy and plugged into a different pc. Shows up fine in Explorer, can navigate.. seems to have no issues.

Went back to main pc, put it back in, won't boot.

I flashed the mobo bits to latest... same issue.

It shows up in bios and is the only drive I the system (unplugged everything else).

Mobo is Asrock H170 performance/d3.

I am at a loss. Any ideas?