^^^^^^ what he says, when it happened to me I tried 8 USB sticks before it would flash properly, also it seemed to kill USB drive's, once used on a box it was a nightmare to get even a PC to see the drives to reformat them. I now have two very old drives, a 128MB one with the new bootloader and another 1GB stick with sucmnsee's back up on it and I then power down the box, plug in the bootloader USB stick, wait until I see done on the front and power box off again, plug in the back up USB and power the box on and leave it 15 mins power down box, unplug USB.......and like magic it works again.
P.S. only thing for me with sucm's back up was the MGCamd 1.35 made my box slow but I run OSCAM anyway
P.P.S Also a couple of boxes would not flash from the back USB but flashed grand from the front USB, strange.