Background: there are a couple of things I do that are far more convenient on Linux, so I decided to replace the USB stick I was using with an SSD. I bought and installed the SSD, installed Ubuntu Studio 20.10, and let it install Grub to /dev/sda. Big mistake.
Windows could not be booted. I wiped the Linux SSD, and then nothing booted; Grub expected to find itself on that drive. Booting the Linux installer from the USB stick would detect the ghost of Grub in the MBR, and jump into it, instead of starting the stick. I had to pull out an old copy of SysRescueCd to put something else in Windows' MBR.
But at that point, Windows would still not boot, nor would it repair itself from the repair mode. I ended up having to reinstall Windows through an unnecessarily confusing and pointless set of menus on the install media. That's not what the story is about, though.
With a functional Windows on /dev/sda, I reinstalled Linux again, and told it to put the boot loader on /dev/sdb. This works fine, with the little weird problem that Grub still can't boot Windows 10?
I haven't gone through and tried
set root=(hd0) and chain-loading that, but you know, this is 2021? This should work out of the box.
Despite having a "Dual UEFI BIOS" on my motherboard, it boots Windows in legacy/BIOS mode, and not UEFI mode. So it's not a case of Linux being booted with legacy/BIOS, then not realizing that it needed grub-efi. I think.
At some level, I don't care. If it doesn't work, it doesn't work. The PC is now configured to boot Windows by default, or Linux when I ask for a boot menu and pick the second SSD. And I'm really just grumpy that the installer/Grub can't figure out how to get Windows up and running properly.