So, the unit doesn't write anything to the filesystem in the Mass Storage Mode, while returning no error to windows. And it DOES write new firmwares to the flash (as I understand CURE firmware makes it boot, while the ORIGINAL firmware makes it hang). This seems strange to me, as I would expect the flash chip to either fail completely or just work normally.
Maybe you should try erasing the filesystem (region 48) using the update.txt & ldr.bin method.
What do others think?

Also, it would be good to check if the firmware is able to write to the filesystem by checking GarminDevice.xml contents, but the firmware fails to boot, sadly.