With all the avatars and Netflix and overall slickness, you might have forgotten one long-awaited feature of the New Xbox Experience(tm). Installing games to the hard drive to speed up loading was certainly overdue. I know there are a few games I have that make the disc drive on my 360 sound like a jet engine. It seems to be a good thing that Halo 3 isn't one of them. Installing the game to the hard drive actually increases load times! Wonder how that works? Bungie's Chief Caching Officer Mat Noguchi (there's a catchy title) explains.
So when Halo 3 runs, if a HDD is present, we copy maps from the DVD to the utility partition (on the HDD). Think of it as an on demand install of Halo 3 to some scratch space on the HDD. Halo 3 doesn’t actually know where it’s running from, so it always assumes it’s running from a DVD. This is an unfortunate consequence of new features (namely, install to HDD) being added to the Xbox 360 after Halo 3 shipped. And as a result, it means that even if Halo 3 is already installed to the HDD, it will still copy maps to the utility partition.
Basically, it's having to read and write to the hard drive at the same time, as opposed to reading from the DVD drive and writing to the hard drive. It all makes sense now, doesn't it? Next time you're up for a little Co-Op Master Chief action, pop in the DVD and call it good. And hope your 360 doesn't take off into the atmosphere.
Source: Bungie.net