McHenryB is only partially (ambiguously) correct in his answer. He says you can have EITHER one 2.5" disk OR four M.2 SSD's. This is not true. You can have all 5 slots filled, not either / or.
The Alienware 17 R2 can have up to 5 total drives - ONE slot accepts a 2.5" drive (either HDD or...