said by JoelC707:Data Deduplication: »en.wikipedia.org/wiki/Da ··· lication
Basically they can let each and every user store individual copies of the recordings. In the background it'll only store one copy and the rest are just glorified shortcuts/pointers. It's not to the point of the OnDemand style access where they would just let everyone watch every show for various reasons but it's not as inefficient as you might think.
There's just one problem with that; The studios argue that if there's only one copy of the program being watched by multiple people, regardless if they specifically requested to record that program, then it constitutes a public performance of that work and is a violation of copyright.
This is why Cablevision's network DVR service stores a unique copy of each program for each subscriber who wants to record it. In court they successfully argued that this wasn't a public performance of a single work because each subscriber has their own unique copy of the program and only this copy is streamed to the subscriber who recorded it. In effect being no different than a traditional DVR, except for its physical location.
This is why it was ruled legal and why Comcast will have to duplicate their horribly inefficient system if they want to avoid being sued out of existence by the studios.
It's also why Aero has to have individual antennas for each subscriber rather than just putting up one antenna and connecting everyone to it.
Copyright law today is a minefield and you have to really contort new technologies to not get blown up.