Mira que lo pone bien claro en el nfo que viene con la bios 1.3a
NOTE for SX users:
YOU CAN *NOT* BREAK YOUR SX BY FLASHING WRONG FIRMWARE
INTO IT.
The SX has an integrated "backup" bios. You can go
into it by holding the D-Pad to the left (D-pad is
the small digital pad at the lower left of the controller).
Do not boot the disc, you also have to press 'B'.
You *need* to start the upgrader from this bios. This
is done due to protect you from evil applications which
could otherwise erase your bios. Still, the backup
bios is read only and can't be erased.
So there's absolutely no way to kill your SX. If you flash
something else to it, all it would do is to stop
booting until you boot into the recovery bios.
resumiendo, graba en un dvd la bios y "basura" hasta 1Gb, grabalo en iso 9660 y metelo en la cube, enciendela pulsando la cruzeta hacia la izkierda y selecciona la bios.