I too am experiencing similar crashes with a WDS500G2B0B, using Marvell controller, as a cache drive. System becomes unresponsive and must be rebooted, no errors at all in the logs.
Did either of you confirm that swapping your drives resolved the problem?
EDIT: For me, it doesn't look like the SSD controller was the problem. I removed it from the system, changed USB keys and tried a few other troubleshooting steps but the problem remained. Ultimately, it appears as though the problem was related to Ryzen C states or idle power. I have a Ryzen 1700x and Asrock X470 Taichi (Rev 2.0 bios), and changing the 'Power Supply Idle Control" setting to 'typical current' looks to have resolved the issue. C-States were left on auto. SSD is back in the system and has been stable for 36 hours.