Thanks everybody. Took twice as long as it should have due to a) both drives being the exact same model, so choosing between them in the BIOS required extra attention, b) having had 2 daiquiris before my attempt, and c) forgetting to remove the Windows install USB and going through that installer twice, probably due to b).
Anyway, the Ubuntu drive is now the primary, and Grub picked up the Windows install just fine.
One odd thing, if I use the BIOS boot selector and manually select the Windows drive, it starts to load, then the computer shuts off. If boot to the other drive, and select Windows in Grub, everything’s fine. Not a big deal since it’s doing what I want, just found it strange.