it probably writes and MD5's a giant sparse file with some salt lightly scattered throughout it in order to force allocation of the space or fail hash test.

I’ve wondered that too. If you wrote your own client you could lie about the hashes, so there must be something else to it. I have a farmer going and maybe every 10 minutes the hard disk spins up for what sounds like very little I/O so it’s definitely not checking the entire 100GB plot file.

Chia doesn't rely on empty space; it fills the space with useless data.