Originally Posted by
operator
And he's wrong.
Too true. There's a basic incompatibility between "road" cassette bodies and "mountain" cassette bodies. The "mountain" hubs have a slight recess in the hub that the cassette body fits into. If you put a "road" body on a "mountain" hub, the body will fit, but the cogs will end up too close to the spokes and the derailleur will rub on the spokes.
I had this exact issue crop up last summer due to an on-tour emergency cassette body replacement. It left me without my big cog- but by then we were done with the most extreme climbs. I made it home OK.