Whether to invoke grub-install with --removable.
Unless you turn this on, GRUB will install itself somewhere in boot.loader.efi.efiSysMountPoint (exactly where depends on other config variables). If you've set boot.loader.efi.canTouchEfiVariablesAND you are currently booted in UEFI mode, then GRUB will use efibootmgr to modify the boot order in the EFI variables of your firmware to include this location. If you are not booted in UEFI mode at the time GRUB is being installed, the NVRAM will not be modified, and your system will not find GRUB at boot time. However, GRUB will still return success so you may miss the warning that gets printed ("efibootmgr: EFI variables are not supported on this system.").
If you turn this feature on, GRUB will install itself in a special location within efiSysMountPoint (namely EFI/boot/boot$arch.efi) which the firmwares are hardcoded to try first, regardless of NVRAM EFI variables.
To summarize, turn this on if:
- You are installing NixOS and want it to boot in UEFI mode, but you are currently booted in legacy mode
- You want to make a drive that will boot regardless of the NVRAM state of the computer (like a USB "removable" drive)
- You simply dislike the idea of depending on NVRAM state to make your drive bootable.