If you are recording music on your computer, it is important to have more than one hard drive. Why? Because, although hard drives are made to be read from and written to quickly, they can only go so fast.
If you operating system is busy working out some stuff for you in the background, and your recording software is busy loading some music or samples to play you, and you are asking it to record you as well... This can be a bit much for one drive to handle.
I would recommend always having your operating system on one hard drive, and your music on another. This extra music hard drive could be an internal or an external drive, either is fine.
If you would like to go to the extremes like me - I have one drive for my operating system, one for my plugins, one for my recording, one for my samples/virtual instruments, and one for my mix downs. But you do not need to go that nuts! I do this because I have had many hard drives die over the years, so I am spreading the load, spreading the risk, and also benefiting from super low latency as a result.
If any of this is going over your head, or if you would like a hand setting up your second or third hard drive as described - Let me know.