@webbhm I've been thinking about what it would take to make a mostly offline data logger and controller that will periodically try to phone home to the mother ship (Particle API, local server, or whatever) and upload its sensor log.
The SD cards we talked about would be one option for storage, but I'm not so sure about the cost benefit in code complexity, hardware, and ridiculously over-provisioned storage capacity. Using the record format in the gist I linked to above, 24 hours worth of DS18B20 + Si7021 + BME280 data recorded at 20 minute intervals only took 6691 bytes.
By my calculation, at that rate, it would take about 400 years to generate 1GB of data. One year would take about 2.5MB, a month would take about 210KB, and a week would take about 50KB. My present record format is meant for easy human readability including units and full sensor names. I could easily shrink it down to 4543 bytes per day (1.7MB/year, 150KB/month) without affecting readability much, and a packed binary encoding could go a lot lower.
All that is to say, SD cards might be the wrong way to go. Perhaps we should look at things more like this 32KB I2C FRAM chip (fast like RAM, non-volatile like flash, and lasts a long time). 32KB would be big enough to go several days without network connectivity.