Using Guix System, the bootloader declaration doesn't currently seem to support being installed to multiple drives:
--8<---------------cut here---------------start------------->8--- (bootloader (bootloader-configuration (bootloader grub-bootloader) (target "/dev/sda") (terminal-outputs '(console)))) --8<---------------cut here---------------end--------------->8--- I suggest to change it so that target can be passed a list like --8<---------------cut here---------------start------------->8--- (bootloader (bootloader-configuration (bootloader grub-bootloader) (target '("/dev/sda" "/dev/sdb")) (terminal-outputs '(console)))) --8<---------------cut here---------------end--------------->8--- This is useful when trying to boot a degraded RAID array. Maxim