Same issue.
Rock 5B boot with SD Card only if no SSD NMVe M2 plugged. When i plugged it, boot seems to fall in a infinite loop with the BLUE Led blinking.
@eliotstock, did you solve your issue with it ? how ?
I read everywhere that’s a possible power supply issue, but i use the official one (35W : https://fr.rs-online.com/web/p/alimentations-rock-sbc/2436356)
My SSD : Crucial P3 M2 2280 4To… seems already use without trouble by a user (ROCK 5B - list of supported NVME)