using nixos-anywhere

hosts/buildbox/configuration.nix

@@ -17,10 +17,12 @@
     inputs.home-manager.nixosModules.default
   ];
 
-  # Bootloader. SeaBIOS/BIOS boot via GRUB installed to the whole disk
-  # (writes boot.img to the MBR, core.img to the EF02 partition).
+  # Bootloader. disko derives boot.loader.grub.devices automatically
+  # from the EF02 (BIOS Boot) partition in ./disko-config.nix, so we
+  # only enable GRUB here — do NOT set grub.devices, or it will merge
+  # with disko's value and trigger the "duplicated devices in
+  # mirroredBoots" assertion.
   boot.loader.grub.enable = lib.mkDefault true;
-  boot.loader.grub.devices = ["/dev/sda"];
 
   nix.settings.experimental-features = ["nix-command" "flakes"];