Tapped the accessory port on my CPAP and the data just kept coming

I went down this same road with my machine about a year ago and it changed how I talk to my sleep doctor. Instead of eyeballing my mask fit, I could show him actual leak rates from bad nights and we adjusted the pressure range based on real data. He was actually interested in the printouts I brought in. For the logging problem, I skipped internal flash entirely and hung a cheap microSD breakout off the ESP8266. Writes go to the card, so no wear worries at all, and I pull the card once a month like I used to do with the data card anyway. One thing that bit me was ground loop noise making garbage frames show up around once an hour, fixed it by powering the board from the machine's own USB port instead of a separate wall wart. My unidentified flag turned out to be the humidifier level once I emptied the tank and watched it change. Try flipping that knob on yours and see if your mystery bit moves.

I did somthing similar with my machine but stopped at the dashboard part and went sideways instead. My setup pushes the readings to a little server box I allready had running in the closet, so the ESP32 itself never stores anything. Wifi drops mean nothing gets logged for those minutes, but the history lives somewhere else and I never once worried about wearing out the chip's memory. If you have an old laptop or a spare machine around, that might beat the buffering dance entirely. That mystery flag you mentioned, I chased one of those on my unit for weeks. Turned out to be the humidifier status in my case. It would flip whenever the tank ran low, which actualy ended up being useful becasue now I get a nudge to refill it before it dries out overnight.

One thing I would suggest from expirience, keep a written note of what your normal numbers look like for a couple weeks before you change anything. My first month of data looked alarming untill I realized my baseline just runs higher than the forum averages everyone quotes. Now when something looks off at 3am I actually know it is off instead of panicking over nothing.

Before the checksum drove me nuts I used to run the raw dump through a spreadsheet and color-code the bytes that never changed. Found the header and frame length that way in an afternoon. Back in the day we did the same trick with a laptop and HyperTerminal tapping into equipment ports, and honestly nothing beats just staring at hex until patterns jump out. For the wear problem, an old trick is to only write when a value actually changes, not on a timer. Pressure readings barely move overnight so most of your samples are duplicates anyway. What speed is that port running at? Mine chattered happily at one rate until the machine kicked into a different mode, then the terminal turned to garbage until I matched it.

This is way over my head but I want to try it. I have one of those machines on my nightstand and never knew that little plug on the side did anything. Does the machine care that something is plugged in there, like does it stop working right or complain on its screen? Mine beeps at me for everything so I'd be scared to touch it.