Sounds like 42 has a fault to me if their horn beeps.
Don't confuse record and save which are two completely different things and is why the size of the media card is largely irrelevant.
If enabled, 'dashcam' is always recording, but it only retains the last 60 minutes, which is then effectively overwritten. For a clip to be saved, you need to physically save it, either with dashcam save clip button, or, if enabled, pressing the horn. A serious external event will also cause a clip to be saved - definition of serious external event seems to be if airbags are deployed.
So, unless you will save an event, you are using no media space other than to hold the rotating 60 minute recording. It use to be that a car not fully asleep would still be retaining only the last 60 minutes. Not sure if that has changed now, but it use to be that if you wanted to view that 60 minute clip, to stand a good chance of viewing it, you had to remove the media card from the car pretty much immediately otherwise the footage would be lost. If you save a clip, it is retained until deleted, so can fill the media drive.
Sentry is a little different. Sentry events are saved automatically but they are also deleted automatically after a period of time when the car thinks the media is getting full - lots of speculation as to what is 'full'. But sentry recordings will not fill the media up, but if you leave the car for an extended period of time, then you may find that old sentry events are deleted. That length of time is the only relevance that media card size has, other than a larger media drive will be overwritten less often which will extend the media life.
The only 'grey area' for me, is the transition between a drive and sentry, when a clip will be assigned to either dashcam or sentry - not bothered to investigate the definitive circumstances as to which one it will be assigned to. In the old days, we had to removed the media drive to view clips, so it was more of an issue as clips would end up in different media card folders - a bit less of an issue now with the in car viewer.
tl;dr - if you want to keep an drive event, you need to manually save it either by the dashcam button, horn (if enabled) or a serious event...
Ooh, and don't save an event too quickly after it has happened. It may missed the event. Likewise, saved clips are not contiguous, and there may be a few second gap every 60 seconds or so in the recording. Guaranteed to be the few seconds that something interested occurred...
