rEFInd is an excellent UEFI boot manager, that does almost everything for you. It is able to automatically find all bootable partitions on the available internal and external disks, and detect the operating systems on them.
rEFInd boot menu
When not using the automatic scan however, we need to add the OS entries manually.
Examples are available in the config file, but, to add an entry from an external hard drive, there’s a small trick involved, that may not be apparent at first.
A volume parameter needs to be specified, pointing to the exact boot
partition of the external disk. All parameters coming after the
volume definition will work relative to the new disk, while
parameters coming before will assume the original disk.
menuentry "GuixSD" {
icon /EFI/refind/myicons/os_guixsd.png
volume 12345678-9abc-def0-1234-56789abcdef0
loader /EFI/Guix/grubx64.efi
}
It should be noted, that the volume ID above is not the file-system one, but
the partition PARTUUID, that can be retrieved using blkid.
Incidentally, to get a screenshot of the boot screen, simply press F10, and the image file will be saved in the rEFInd root directory.
The boot menu screenshot above is based on the “Minimalistic rEFInd theme”, while the icons are from “rEFInd theme Regular”.