If it's a Maxtor, it certainly wouldn't surprise me if it is going belly up. They haven't been making the most durable drives as of late. Anyways Xbox-Scene has an error code definition somewhere on the site. I will do some research for ya and post back with my findings. As a last resort you might try re-installing Slayer's and see if that happens to fix the problem. EDIT Sure enough, this is what I found when searching through the faq forum over at X-S:
Error Code 7-
"HDD timeout this basically means that the xbox was able to locate the hdd but its not responding. This error usually is a serious hdd malfunction junk the hdd and get a new one."