[jyl]

Swapped old OEM SSD back in, MBP boots and runs normally.

Irritating thing is the USB enclosure does NOT work with the new SSD, ONLY with OEM Apple PCI SSDs. The documentation warns very explicitly not to put the new OWC SSD into the OWC USB enclosure and that the OWC SSD may be damaged. I don't know why.

I'm thinking I need to rummage around, find an external USD HDD, wipe and format it, make it a bootable clone of the old OEM SSD, then repeat my original steps. Make sense?