While slightly costly, you can buy a 4TB 2.5" SSD in a normal thickness drive. You can not do that with a HDD, the only 4TB 2.5" HDD I am aware of is a 15mm thick one which does not fit in, well, pretty much anything, not laptops, not most bays, nothing, they are only usable as an external drive (but it's useful that way, I have one). I believe this is the first time the capacity crown goes to an SSD at any given time (at least in the consumer space -- in the server space the 16TB 2.5" Samsung SSD and the 60TB 3.5" Seagate are both out of this world but so are their prices too).