Well, just debricked my E3372h-510 test pointing it, uploading usblsafe-3372h.bin with balong_usbdload and loading 21.110.99.02.00 downgrade with Huawei Flasher v2 (before extracted HMF with Huawei Flasher v1 and removed first comment line from HMF file).
You need connect those pins: http://s.kaskus.id/images/2015/12/29/8228477_20151229061027.jpg (or just boot to USB “GND case”)
When you plug the modem, it won’t blink, light is off but UI Interface is detected by PC.
Then:
balong_usbdload: https://github.com/forth32/balong-usbdload/blob/master/winbuild/Release/balong_usbdload.exe
usblsafe: https://github.com/forth32/balong-usbdload/blob/master/usblsafe-3372h.bin
Usage: balong_usbdload -p8 usblsafe-3372h.bin
(-p8 is COM8, as -p4 should be COM4)
Finally, instead of green quick blinking, modem will be cyan blinking very quick. Then use modem flasher v1.6 to extract, notepad to edit and finally modem flasher v2 with go-hldc ( http://www30.zippyshare.com/v/aee8MgJa/file.html ) to flash HMF — (Modem Flasher v1.6 is easier to find).
It should work to flash any E3372h with their respective firmware.