Setup: 3 HDDs in a legacy BIOS PC (Not EFI.) Windows is on sda, encrypted Linux Mint Xfce 19.2 is on sdb, and encrypted Linux Mint MATE 19.2 is on sdc. To me, this seems like an obvious way to use 3 HDDs - put a separate OS on each one, and everything's compartmentalized - you choose whichever one you want at the Grub menu. But maybe it isn't obvious to other people. I installed Windows first. Tested it, no problem. Then I installed Mint Xfce 19.2 on sdb, encrypted with LUKS. Both Windows and Xfce showed up in the Grub menu, no problem.
Then I installed Mint MATE 19.2 encrypted on sdc. It does not show up in the Grub menu. I don't know how to get it into Grub without kicking Xfce out.
sudo update-grub does not work. os-prober does not work. Nothing I've tried works.
I can't believe that nobody has ever wanted to have more than one encrypted OS on their computer. I've seen screenshots of people with 7 or 8 versions of Linux on their computer - are they just not encrypting them? Even if it's a laptop? There has to be a solution.
I installed a program called Grub Customizer, hoping it would offer a simple solution, but it turns out there are no instructions on how to add an OS with Grub Customizer. In the GC GUI, I added the MATE 19.2, got an error:
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error no such device error: unknown filesystem error: you need to add the kernel first