Lime Technology's unRAID Server OS offers a lightweight package for a quick NAS setup with an innovative software RAID driver. Unlike traditional RAID modes, unRAID doesn't do any striping or mirroring. So you don't get the performance benefits of striping or the redundancy of mirroring.
Instead, unRAID stores parity data on one disk in the array. This configuration allows a single drive failure to be rebuilt, and allows you to add another drive of any size or speed (it must be smaller than the parity drive) to the array at anytime.
unRAID is built on Slackware 11, one of the oldest and most mature Linux distros out there, running a fairly recent 188.8.131.52 kernel. unRAID comes in three flavors: