> > (define %operating-system
> >   (operating-system
> >    (host-name "image-test")
> >    (bootloader (bootloader-configuration
> >                 (bootloader grub-bootloader)
> >                 (targets '("/dev/sdX"))))
> >    (file-systems (cons*
> >                   (file-system
> >                    (mount-point "/")
> >                    (device "/dev/sdX1")
> >                    (type "ext4"))
> >                   %base-file-systems))))
> 
> Do you correctly substitute /dev/sdX for your real device name (e.g.,
> /dev/sda) ?

I did try with that too, but it failed with the same error. Today I
almost accidentally found out that this error was caused by a corrupt
store item and was fixed by running `guix gc --verify=contents,repair`.

The error message could certainly be improved (or rather, a error
message about a corrupt store could be added), but I didn't find the
actual failing check so that I can't send a patch.