I backed up my X360 HDD, rebooted and used the X360 dash to 'move' my profile to a MemPak. Then restored to the X360 HDD. Profile now on HDD & MemPak!

Works fine when one or other plugged in (I'm careful not to have both connected at the same time. Not sure what would happen.

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But I don't use XBL! It may have additional checks on profiles! (Such as sensing the local gamerscore is suddenly less than the server held one!)
I guess you could also achieve this by simply unplugging the HDD and doing an XBL Recover Account to the MemPak.

Thinking about it a little more, while this may not work still, I don't believe M$ could ban you for this, as you could get into this situation quite legally. Say you brought an X360 HDD, recovered your XBL profile to it, and installed Final Fantasy - which I hear takes up a lot of disk space. You then might purchase a 2nd HDD, for other XBL DLC, and recover your account to that HDD also.
yeah, but what would get rid of that problem would be to always have the mem card in the system for your profile and the HDD for storing games.
this gives me an idea, I'll take and backup my HDD and then move the profiles on the mem card, then restore the HDD and see what happens.
here's my reason to wanting it... I mainly use my 360 for media center, and i'm getting a seond one for the bedroom, to get rid of the computer off the tv... i want both mine and my girlfriends profile so that we don't have to move gamer tags everytime we want to either move in the bedroom or to play the same game on two different systems.