507 just means there is no OS on the device. Not sure what's causing the loading operation to fail. It could be a computer issue...make sure you're not connecting to a usb hub. It could be a device issue (get a new BB.)

Have you tried this:

How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ

and then this:

What to do next if Jl Cmder will not connect and wipe - BlackBerryFAQ

Wiping with JL Cmder (the first link) simply gets your device to the 507 screen. I suggest using the second method and see if that will reload the OS to the device.