Much as I agree with not upgrading firmware without good reason.... You are very unlucky to get a failure. I have an S5 Controller here that I play with and have probably upgraded 20 or 30 times without problem, this includes loading all of the different S7 firmware versions onto it to have a quick look at what was in them.
You should be able to recover from the SD card with an image or if that fails you can connect a Terminal, although this requires taking off the BBB and soldering some connection pins and having a 3V TTL to USB converter.
Rich
Thanks for the suggestion, I have a 3v TTL to USB converter but no solder unfort. I do have an SD card however, will try just dropping the contents of the upgrade tar.gz onto a fat32 partition but in case that doesn't work do you have any advice on how to proceed?
Thanks
You might contact bitmain. I'm not sure if they have the ability/or what it takes. But on S3's they would send out info to un-brick a S3 via serial/usb.
I would at least ask them as it does not hurt. I have not seen anyone mention this so not sure what their official response is. Also if in warranty defintally contact them as they will fix it, but downside is you lose day's of mining compared to fixing it yourself.