Question: Can you push [Tenser's Floating Disk](http://www.5esrd.com/spellcasting/all-spells/f/floating-disk/) so that it doesn't just hang out behind you? If so, how hard would it be to push?

Answer: 
RAW, the [text of the spell](https://www.dndbeyond.com/spells/floating-disk) says "No":

> 
> The disk is **immobile** while you are within 20 feet of it.
> 
> 
> 

Immobile is pretty clear (emphasis mine).

That said... I don't see any reason why a DM should not allow the spell and its cargo to be moved around at the expense of an action by the caster.
No
==

It isn't described as having any weight, but it is described as immobile in it's description, at least while it is within 20 feet of you.