So, you're not ready for rack-mount storage yet, but you need more than just an external hard drive to serve a few clients. It's time to look at Network Attached Storage (NAS).
There's a disk configuration to suit almost every situation, from two to 12 disks. Firmware tends to be shared between models of the same age, so feature-set tends to remain common within the one brand. Performance and capacity will of course change depending on how many disks it supports, what processor is inside and how much RAM it has. You'll also need to determine whether you need features like dual Ethernet for failover before making your selection.
If you're in the market for one of these beauties, here are our favourites of the NAS systems that we've seen thus far.