getting past white screen of death

[Sarcany]

Mr Downtown wrote:
I took the SSD out and put it on a USB connector. My laptop can't mount it, though it will mount my old (pre-November, smaller) SSD.

Could the system update have somehow bricked the SSD?

If your laptop isn't running at least macOS 10.13, it won't mount the APFS volume from a High Sierra boot drive. But the Disk Utility should still see the SSD. Did you run the DU to see if the drive was accessible?