All hard drive lights are flashing

I have a dell 1950, running Linux. Its been reported that an application I have running on it is very slow right now. Went to the server and noticed all the Hard drive lights are flashing (at least the lights on the bottom, not the top).  I've had other servers to do this as well, it usually stops after awhile, and people report they are slow during this time.

Any knows what it might be doing?