I have a Pi 3B+ that wont boot. It's a "new" unit that I received as a gift a few years ago and am just now taking it out of the box and trying to use it.
I've tried several OS images, but they all behave the same way... a rainbow screen with a yellow lighting bolt in the upper-right corner. The power LED flashes slow4 times and then fast 4 times.
From what I've gathered, this indicates that the firmware needs to be updated. But, I can't get an OS to boot in order to update the firmware. All of the articles I have found talk about updating the firmware on an already working OS image. I can't even get that far!
How do I update the firmware if I can't get an OS to boot at all?