The xbox 360 hdd is upgradeable. I replaced my grandsons 80 with a 320 and then put the 80gb in a usb enclosure that he uses to backup his games with (16gb) plus keep music on. which is only 25gb total but you get the idea... a harddrive is a harddrive.http://www.amazon.com/Xbox-360-320GB-Hard-Drive/dp/B005PSTV2K/ref=sr_1_1?s=videogames&ie=UTF8&qid=1344048733&sr=1-1&keywords=xbox+360+320gb+hard
edit - since you got it to work, at least try to backup your data in case the drive does have a problem.
also, it could be a bios firmware issue. update your bios. you can get the files and instructions you need from acers website. I beleive you need a usb stick to do it but it does fix a few bugs.