This damn game is 'forcing' me to buy a hdd, but before I buy one second hand on ebay are there any reported problems with 360 hdds? Obviously you would have to be a bit daft to buy a second hand 360 but are the hdds ok?
I was in the same situation, bought a 2nd 360 hdd, took it out of the shell and it worked perfectly, although considering it's supposed to be 20gigs, 360 informed me there was 14gigs free. The 360 recognized it immediately and could play firefight fine now, haven't played coop yet bit should be fine.
Also installed reach and downloaded a couple of demos with no issues.