Given your requirements, you might want to research a Windows Home Server box, install two hard drives, and enable "folder duplication" for the music folder. HP makes their MediaSmart series, and Acer has a stylish H340 WHS box. You can also buy a copy of WHS (you would probably want the 2003 version, based upon a locked down version of Windows Small Business Server 2003) and build your own computer:
http://www.wegotserved.com/As long as you don't run into a defective particular model, most hard drives are probably quite reliable. 5400 rpm is probably fine for external media storage, and typically will be quieter and generate less heat than 7200 rpm external hard drives.
Externals do differ in terms of warranty (possibly as little as 1 year), though, so you would want to research that.