Just a thought I had.
I know Catapult restricts the items it can use on to items weighing 1-5 lbs. Tenser's floating disk doesn't really have a weight mentioned (not sure if we can assume it's weightless though).
Even if, what if we put some weight on it? (Is it and everything on it weightless. Most assuredly not or it wouldn't have a limit right?) For instance what if we put something small like 2-3 lbs on it, wrap it in a blanket all weighing between 1-5 lbs kind of like a pie.
The other part of this which I'm willing to bet will be the main response is it's considered immobile while the caster is within 20 feet of it. That by itself is fair enough. However, could you do it while you are beyond 20 feet. After you are more than 20 feet apart it begins to follow you. Thus it is no longer immobile. At this point could you conceivably force it back with catapult?