... and the laptop cannot boot from the SD card slot as it's connected over PCIe and not USB. Thus is not available as a boot device is the BIOS/EFI boot menu.
As I've already built the RAID on full drives, not partitions, and my data is synced to the RAID, I've painted myself into a corner. I've partitioned the RAID as well.
I'm not sure how to get myself out of this one. I guess I could just get a flat USB drive to keep permanently plugged in.
Ugh. What a PITA.