> should I describe my Bios Grub filesystem in '(operating-system (file-systems > (list ...)))' or just > only in '(operating-system (bootloader ...))' section?
You do not need to mention grub in "file-system" section of config. You have to mention just the device "sdX" NOT with the partition "sdXY", in the "boot-loader" section of the config. Assuming the device is formatted as GPT, you should have either "BIOS Boot Partition" (for BIOS systems) or "EFI Boot Partition" (for UEFI systems) on the device. In the "boot-loader" section of config; if you use "grub-bootloader", it will detect the BIOS Boot Partition and install grub on it (or) if you use "grub-efi-bootloader", it will detect the EFI Boot Partition and install grub on it. > If I need to describe grub file system, so which type should I specify? No need. See the above. > If I do not need, how grub will know to boot from my root partition Linux > filesystem on /dev/sdb2 ? The grub uses file-system drivers to detect file-systems and look for grub.cfg. As the grub.cfg (in /boot/grub of /dev/sdX2) was generated by guix based on the config file, it has information where to look for kernel. Regards, RG.