Ive flashed 2 times now, and from my experience your oldest usbstick has the most chance to trigger the exploit, i have like 10+ usbsticks but my oldest one was the best for this. Same for installing the cfw after that, just one usbstick, or cardreader with an old sd card in it. I think it got something to do with the read write speed.
Hope this helps for you too.
Also, you need to be very carefull with the data you try to put in your ps3, always use "eject usb" with right click on the drive before pulling it out.
And last, just put only the hex file on an empty card/stick in the right usb port, closest to BD.