If a friend. knows you are taking a technology class in college and asks you how a hard drive working

There are two possible cases to that situation:

Case 1). You know the answer to his question.
Case 2). You don't know the answer to his question.

My advice in each case is as follows:

Case 1). 
Using what you know, give him a simple answer to his question. 
Then ask whether he wants to know more, and deal with his additional
questions, to the depth, and in the detail that he asks AND you know.

Case 2). 
Tell him that even though you are taking a technology class, you have
not learned the answer to his question, and you do not know it.