The hour meter is stored in non-volatile memory in the cluster. Meaning, disconnecting the batteries wont reset it.
However, there is a "glitch" in the cluster software that, if the batteries are getting weak and the voltage drops too low during cranking, it CAN cause it to reset.
LLY's seem to do it the worst.
My truck has reset probably 6 or 7 times over 200k miles. Ive had the batteries disconnected much more times than that. And ive left the truck with batteries disconnected for months and it didnt reset. Yet then I would notice at random times while im driving (and the batteries havent been disconnected in forever), ill look down and the hour meter says 1.6 hours or something.
There is no common thread with the hour meters resetting... its NOT "when you disconnect the batteries, it resets"
and while we're on the topic of instrument clusters, if you suspect a truck your buying has "odometer fraud", you can tell if the cluster has been replaced. On your build sheet, GM tracks serial numbers of lots of main components that each truck leaves the factory with. ABS modules, engine serial numbers, trans serial numbers, instrument cluster serial numbers, airbag module serial numbers, and much more. So get the dealer to print your build sheet, look at the instrument cluster serial number on the build sheet....and then look at the actual serial number on the instrument cluster itself. If they are different, then you know the cluster has been replaced. If you have a tech 2, you dont even have to remove the cluster to check the sticker for the serial number....you can just ask the cluster what its serial number is with the Tech 2.
ben