In order to increase the capacity of your UPNP library then you normally have to just upgrade your entire array, which can be rather costly with NASes using 4 disks or more. Although hard drives are cheaper today (you can get a 1.5TB drive for less than $100), you have to buy at least four of them at once.
If you don't have money flowing out of your wallet, then you can purchase a single hard drive, slap it in an enclosure, and connect it to the USB or eSATA port of your NAS. But then the problem arises about how to get the UPNP server to see your video and music files. The answer lies with the use of Symbolic Links!
Read more at OCModShop.