I just recently got a new 360, and lately I’ve been having some trouble with using theatre mode in Reach. It keeps telling me that an Xbox 360 hardrive is required to watch a film, which (being mentally challenged in the ways of technology) doesn’t make much sense to me. I checked my console settings for a storage unit, and apparently the system calls it a Memory Unit. Again, not sure what it means, but I would very much like to make use of theatre mode once again. Any help or suggestions would be greatly appreciated 
Well I can help you out if you have a normal 360 not a slim.
Do you have a box thing with HDD(That’s what it says on mine) on the left side of your xbox. If you don’t then thats your problem.
Go on Amazon and search Xbox 360 hard drive then if you buy it put it in your xbox.
Also memory unit is a small data storage thing that enable you to save games file not a hardrive.
Here’s a pic of a hardrive.Hardrive
My system is a slim. Apologies for not being more specific.
Well, that’s not right, cause all slim 360s come with hard drives. If I were you, I’d have someone check to see if there’s a hard drive in it, or find a Youtube video and do it yourself (it’s not hard)
That is odd…
Shouldn’t be happening on a Slim. Are you SURE that’s what you have?
Wait, the slim doesn’t take Memory Units, only the older systems. Is this the system you have? http://microsoftxbox360elitesystemgameconsoleblack.com/wp-content/uploads/2010/11/black-xbox-360-elite.jpg
The 4GB slim is not considered a Hard Drive. This is an issue Microsoft/Bungie are aware of and hopefully a fix is coming. Until then I suggest buying the Xbox 360 250GB Hard Drive or seeing if a flash drive would work.
Figured it out; I had to transfer the hardrive from my old Xbox to the new one. Thanks for all the helpful suggestions, mates