I had the same business happen with my attempt to flash. I got around it by forcing bootloader mode (pwr, ok, reset)
Why flashing with your pc... risking power outage, windows bluescreens..
When you take the nbh file and put it on your storage card en rename it to HERMIMG.NBH
After a hardreset it autoloads your nbh file without your pc!
Ofcourse watch your battery BEFORE you flash!