External hard drive for expanding iTunes library?

Speed is not an issue. Even the slowest hard drive in the world is way fast enough for music playback.

Since this old thread got updated, I went with the Thecus 5200 Pro with 5 750gig SATA Seagate drives in a RAID 5 configuration. So I have 2.7 Terrabytes of usable space which is already filled. This NAS is 2-3 times faster than moving files from a MS windows box, it's truely amazing getting 30% utilization on a gigabit interface... When 1TB drives drop in price I will switch to those. The 1.5Ghz Processor and the 512meg of Ram are plenty fast to run the latest version of Slimserver with MySQL.

To backup my Nas I have a PC which will be running FreeNas or MS Windows Home Server running JBOD via the OS.

As Herman pointed out that the slowest setup is still no problem for music playback.

speed can be an issue. if your music server is also a repository for your other computers in the house and you have multiple streaming going on, i think it could make a difference. the disk is the slowest part of the system (if you are directly connected to your amp and are not using the network to stream the data) and if you can tweak better performance for reads and you use around 3/4 of your total disk space (compared to raid 1,10), why not use raid 5.

I have no way to prove this but I believe the consensus is that if your PC has so much running that the disk reads can't keep up with something as slow as digital audio then the sound is being compromised anyway by all of the background activity.