Gear4 Blackbox haxoring

So, I owned this cute boombox and.. yes, it has a USB interface ;-)
So, I downloaded the OpenMokoDFU.zip driver and plugged in the USB cable into my Gear4 Blackbox and switched the Blackbox ON ( how to find a USB port on a Blackbox ). Windows detected it with PID/VID 1234/0001 so I patched the driver to match the PID&VID and installed the driver. It's convinient to make another copy of driver, for these PID/VID also: 1234/ffff - you'll have to install this copy of driver when you acccess the Blackbox for the first time after it have been switched ON with USB connected. Then I used the dfu-util to get the firmware from the Blackbox: dfu-util.exe -t 512 -U original_fw.bin. I found the string "Gear4-Blackbox" (bluetooth name of the device) inside the firware, replaced it with my own string and attempted to write the modified firmware. I failed. Maybe there is a checksum inside the firmware that has to be recalculated before the firmware upload. Anyway I failed to upload the original firmware too. Will try again in my freetime...

Here are some interesting strings <>:

CSR-dfu2
LOADER=3kEnhancedFl2v0Blu..., STACK=3kEnhancedFl2v0Blu...
CSRbcfw1
Gear4-BlackBox