By common hard disk im assuming you mean the server's C: drive right? As for the rest of your question, its a bit vague so I'm going to take a few guesses here.
If it's not yet password protected, then yes you can find a way to put your own pass on it or change permissions so that the admin can't. The problem is, they are the admin. Read that again. So whatever you do to it, they can get rid of and change it to how they want it, the most that will happen is that your causing a headache for them. If nothing else they'll just swap hardware or something man, theres a reason sys admins are like gods on their own network. They have the POWAAAA!!!!
If theres already protection on it that prevents you from accessing it, then it depends on how they did it. What you'll be looking for is either an admin password or a way to escalate your priveleges so that you can access it. The key is to do something the sys admin won't notice (like getting his pass from a sticky note on his desk) and not changing anything no matter how broken it is, the longer you stay under the radar the longer you will have access to this disk/drive/blow up doll. Just taking a guess on the last one
Post back with more info man and we'll see if we caan't help more. Some better formatting and longer sentences might help as well
