I’ve finally fixed this problem: <http://bugs.gnu.org/18061>.
Not perfect, since it requires users to type an extra command, but that
seems acceptable to me.

I would very much appreciate new tests and feedback.  Here’s a new image
with this fix:

  http://www.fdn.fr/~lcourtes/software/guix/gnu-usb-install-20140723.x86_64.xz
  http://www.fdn.fr/~lcourtes/software/guix/gnu-usb-install-20140723.x86_64.xz.asc
  SHA1: 7d277ed9a0927fd08e211125eca9a8bea98e7c3e

(For 32-bit, you can rebuild it with the usual command.)

Thanks,
Ludo’.