Functionality of Xbox : Hardrive Issue

[vodanh]

My xbox does not boot up anymore, my friend fried it somehow. It may be a power issue but I really suspect the hardrive. It creates a loud beeping noise (or a zzZZ sound from over spin ) when connected. My question is does the hardrive hold any OS related files or purely save files ? If the xbox boots and runs everything from bios and the hardrive is just for storage i can simply try to replace the hardrive, but if there are files on the drive required to run the xbox then I'll have to copy it off someone elses xbox.