The even later ones will either do a Wifi downlink every time they hit home base or for the more nomadic aircraft will use a mobile phone to phone the data in following arrival.
The number of flights recorded therefore is unlimited as the data is stored on a company server.
Does anybody know what the sampling rate is?
How long is a bit of string, usually base four so some things ... (very rough examples) will be four times a second (AoA), some 16 times a second (momentary action switch) some once every 128 seconds. (CofG)
would there be any way to prove the recorded data was incorrect?
In general to prove the validity of the data is a time consuming task.... depending on what precise parameter you wish to verify. At its simplest you can go an function test it, then see if the recorded data matches with what you just did.
does it only store what is on the black box
Often a slightly different (usually expanded) set of parameters are recorded with near a thousand not unusual on modern aircraft.