Easy way to add more HDD

I would not change my opinion regarding RAID5/RAID6. They are not so reliable as all thinks and they are not a silver bullet. And they obviously more expensive solutions.
I do not want to repeat the whole RAID vs No RAID choice thread, you may read it yourself.

We all makes decisions ourselves, so calculate, think and act as you believe is a right way for you. The warning regarding RAID will remain for others.