Update on my experiment. I am extremely familiar with installing Linux, and as I said above, the external SSD I configured with Mint was set with MSDOS partition table, which it is. But I also explicitly told Mint (using "something else") to install the GRUB bootloader on this external SSD... but it did not. GRUB was installed into the /boot/efi partition on the Windows internal SSD instead. This is why I get the grub> prompt when booting with the external SSD removed.
If these drives were both internal, this would be fine. I could choose the Mint drive to be first in the boot order, and GRUB would allow either OS to be booted... the outcome desired by the OP. I don't want to have the external always plugged in, so the Windows Boot Manager is set to boot first again, and I have to use the Boot Menu to select Mint if I want it instead.
It seems that Mint decided it knew better than me how to install itself, although I don't necessarily agree with that. This external drive will not boot on another computer now because it has no bootloader, except on my Windows laptop. This explains at least one reason why the OP's original advice to disconnect the Windows drive before installing Linux to a separate drive is a good idea. However, it is probably also true that Mint only boots at all because it created a UEFI bootloader... otherwise, I would have needed to change my BIOS settings to Legacy, or both UEFI/Legacy if that option is available.
If these drives were both internal, this would be fine. I could choose the Mint drive to be first in the boot order, and GRUB would allow either OS to be booted... the outcome desired by the OP. I don't want to have the external always plugged in, so the Windows Boot Manager is set to boot first again, and I have to use the Boot Menu to select Mint if I want it instead.
It seems that Mint decided it knew better than me how to install itself, although I don't necessarily agree with that. This external drive will not boot on another computer now because it has no bootloader, except on my Windows laptop. This explains at least one reason why the OP's original advice to disconnect the Windows drive before installing Linux to a separate drive is a good idea. However, it is probably also true that Mint only boots at all because it created a UEFI bootloader... otherwise, I would have needed to change my BIOS settings to Legacy, or both UEFI/Legacy if that option is available.
I agree! And it sure isn't me!I sometimes wonder if anyone truly understands the whole UEFI/BIOS/GPT/MSDOS/ CPM/LEGACY blah. What a mess.