If you load the later fw gupdate.cgd onto a microSD card, you do not need to be in preboot. Simply place it in a Garmin folder on the card, turn the unit off and then insert the card. Rebooting the unit will allow the fw to be loaded.

If you wish to do it in preboot, you need to have usb access to the unit which you don't seem to have going by your post.

Are you sure that this unit isn't being seen via usb is due to a hw fault? Hard resetting a unit and/or clearing non-vol sometimes will solve erratic behaviour. Many things can be done via the card including removal of files/folders and voice and map updates etc using script commands with ldr.bin, but it's a clumsy way to have to do things.