It failed because it appeared to have locked up (but it seems it was merely starting the update process, at which point it is unresponsive), causing me to pull the plug after a couple of minutes.

If I'm being honest "a couple of minutes" probably translates to just slightly more than a minute. But in my defence I had no reason to expect that the BIOS update had actually started, as far as I knew it was just a "normal" lockup.

I had no way of knowing that pulling the power at that point was in anyway dangerous. And it isn't as if I'm an in-experienced user...