I love the idea of delegating service seperatly. So rather than habving raid built into a server I would like to see it in a rack mountable exteranal inclosure. Something you could rack above the server.
USB would be fine for connecting now that it’s as fast as most sata connections.
As computers evolve I would think everything will be seperate and interchangable. For example a laptop would have it’s os on a flash drive and servers will most likely have external hard drives.