I'm posting this here in case anyone in the future bricks their SENA headset during a firmware update, like I did. I found a report from someone else who unbricked theirs, from five years ago, and the process worked for me as well.
So -- I tried updating the SENA headset's firmware, and the firmware update program locked up; I had to CTRL-ALT-DEL to get it out of its lockup loop. Upon doing so, I found that my SENA SMH5 was dead. Dead as a doornail dead. Won't turn on, won't anything. The charging light would show up, but other than that, absolutely nothing. Sigh.
I googled extensively, looks like a LOT of people have had this problem over many years, at least back to 2012. Some people say using the RESET button works, some say it doesn't, etc. Most people said the only solution was to pay to ship it back to SENA and pray it was still under warranty.
Well, I found one guy who had the exact same situation as me. See this post
, from Matt, on Dec. 27, 2012. He said he had the exact same thing happen, and was going to send it back, but he decided to try again, one last time, before doing so. He wiped and reinstalled the firmware update program off his computer, and simultaneously he moved his USB cable from a USB3 port to a USB2 port. And it worked! Not only did the SENA come back to life, but it completed the firmware update procedure.
That sounded absurd to me, but I thought -- what do I have to lose? So I did what he suggested. And, I went a step further -- instead of just using whatever micro-USB cable I had nearby, I went and specifically got the micro-USB cable that came with the headset. That cable is a right-angle cable, and it seats much better in the SENA's micro-USB port than my other cable did. The cable I'd been using would charge the headset, and it recognized and let the firmware update procedure start, but then it all locked up; I don't know if switching to the official SENA cable made any difference, but it definitely seated in more firmly so ... maybe?
In any case -- doing all these things, it worked exactly as it was supposed to. The program installed, the driver installed, and when it told me to power up the SENA... it did! I was shocked. But also very relieved. The firmware went on to update properly, and now the headset is working 100% again.
I don't know which if any of these changes made an actual difference. All I know is that I changed three things, and the headset went from "bricked" back to "perfect."
1) remove and reinstall the SENA firmware manager software
2) use only the SENA-supplied right-angle micro-USB cable
3) plug that SENA cable into a USB2 port instead of a USB3 port.
I like my SENA headset a lot, but I like it a lot more now that it's working again.