Thanks to everyone. I feel like this was so easy, how could I have been confused? But I was. Somehow, I was lost for weeks between the ways that Ubuntu and Gentoo handle grub---and I was trying to install unstable ubuntu with grub 2, so that may explain some of it, but not all, surely.
I used the usbkey. Installed grub on the drive I WANT to be /dev/sda, and, what do you know? It worked. Thanks. Alan
