@michapixel:
Do have a backup copy of the previous working USB for device a? If you do, just plug it in. Does it still work? I don't see how upgrade and restarting, even incorrectly, will brick your device. If you don't have one, you probably should in the first place.
I think using JTAG is over kill. If you have access to the serial port, I think using UART booting is probably an easier approach. Although I have never tried that because there was no needs. Maybe I should try it one day. Good luck.
Edit: Sorry, UART booting won't work on Dockstar. Do you still have uboot working?
Do have a backup copy of the previous working USB for device a? If you do, just plug it in. Does it still work? I don't see how upgrade and restarting, even incorrectly, will brick your device. If you don't have one, you probably should in the first place.
I think using JTAG is over kill. If you have access to the serial port, I think using UART booting is probably an easier approach. Although I have never tried that because there was no needs. Maybe I should try it one day. Good luck.
Edit: Sorry, UART booting won't work on Dockstar. Do you still have uboot working?