Hi, I was hoping to get some help. I just...

Hi, I was hoping to get some help. I just bought my second QNAP but I only needed one hard drive for the time being. The other QNAP I bought I filled up all eight bays at once, what I'm wondering is do I have to have them all filled to start using it, or is it easy to add the other hard drives at a later date? If I can add them later is it simple like just installing them and turning it on?