XBOX 360 Memory Unit

I think I want one of these but can't seem to find them anywhere..

The little research i've done has taught me that they are only now available in 256mb/512mb sizes and that only official ones now work.

Now, I already have a hard drive for storage but I regularly recover my gamertag to my brother's xbox for playing party games and the like. The process takes a while though and of course, I can't copy over saves this way.

What i'd like to do is be able to copy my gamer profile, game saves and xbla games too as I buy much more from the Marketplace than him and i'd like to share my purchases. (I see that capacity will be an issue nowadays with some newer arcade games weighing in at over 1gb)

Now, we both have hard drives and I had thought that they would work as big memory units to transfer everything but for some reason, when I tried this it didn't appear to work. So I suppose my question is twofold. Firstly, are these memory cards available and will they do what I want? And second, can the hard drive which I already do all of this anyway? (preferred option)

Thanks.

Anyone?

The game save and your profile are transferable with a memory unit (as far as I know, it's true that only official ones work) but your XBLA games aren't. Now, if you use your HDD on his Xbox, you can play your XBLA games as long as you're online at the time.

It's a needlessly complicated process.