Hard drives have typically been the go-to storage medium for loading up a NAS with data, but flash storage in the form of SSDs have grown in popularity as NAS owners look for quieter and faster drives (and lower power consumption). The best SSDs for NAS have dropped in price in recent years, allowing for a substantially better price-per-GB.
I last bought 2.5″ notebook hard drives for my NAS mainly because they could be powered directly off their USB Cables (compared to the previous larger hard drives I had which required separate power), but I can see next time around I’m pretty sure I’ll be going with SSD drives.