what data disappears after 7 days on S9?

[Lanco]

and is it that important?  Does not the most important data remain?

[justMongo]

For the S9, the high sample flow rate, and Pressure data are lost after 7 days. They are recorded the SD card only; and only 7 days are kept. 30 days of "detailed data" are kept. The machine holds compliance date for a year (hours used per night).

IIRC, high sample rate data is sampled at 25 samples per second. High sample rate data is important if you wish to dig into the waveforms to glean what happened in the proximity of an event.