I have a Raspberry Pi 3B+, running NOOBS. I'm using it for home automation, and it's important to me that it never fail (or if lightning fries it, that it be trivial to swap in a new one. Downtime is very unacceptable.)
Step 1 was having it boot off the SD card, but moving/to a much larger external SSD drive. I needed the space and wanted to minimize writes to the SD card. I got that to work despite some misinformation from Adafruit's website. But when I shut the pi down, I see activity on the external drive as I'd expect, but then I see the green led on the pi flashing, suggesting it's writing to that SD card. (Or is that read activity?)
What's it doing? I don't want the SD card touched - it's there for boot, period. It's been suggested I mount it read-only. Is that a viable option and how is it done?
My next step is to give it a backup power supply.
What else do I need to do? Since I also want the SSD to last a long time, I'm considering moving /tmp and /var/logs to tmpfs. Are there other steps that should be taken?
Finally, I'd like to initiate a controlled shutdown on power failure. Normally I'd poll the Ethernet carrier signal once every few seconds and code a shutdown if it drops; but I'd like to keep this pi wireless to minimize chances of it frying when nearby lightning strikes happen (which in my area isn't uncommon.) Is there something I can poll to indicate when there's no Wi-Fi signal from a particular access point? The alternative is to rig up something with wiringPi and an optoisolator, which I can do, but it's an annoying approach - more wires and more hardware to fail.