I was reading a post on here somewhere recently, where someone did exactly that. They were running a hard drive in their PC with data on it without the top on :Q. I don't remember their reasoning for it. IIRC, the platter came loose from the motor, at full rotation speed, shot off the motor, and hit the mobo, killing it instantly. Then it bounced off of there and lodged itself in the case window, as they described it.
(This is a bad idea)
And I did take the top off of one of those HDs I have...nothing special in there really. Looked kind of like a fancy record player

. I needed an insanely small T-5 bit to get the screws out though. No stores I went into had any, but I was able to get a little set of bits off Ebay for $5.95 shipped. The guy shipped em quick too, they were here in 3 days.