Thank you - I did try installing grub on both drives, but I didn't make the other drive bootable. That might help.

Before I started using Linux, I'd never touched the boot menu on any of my computers - I just used the drives as presented to me. So this is all new as far as I'm concerned.

I'm finding it very interesting, but it gets a bit demoralising after a while when nothing you do seems to make a difference.