Take a look at FreeNAS www.freenas.org
below is the description from the website. I have been using it for several months. I had an old P3 setting around that I converted into a NAS using this software package. Works great never had a problem.
Welcome to FreeNAS
FreeNAS is a free NAS (Network-Attached Storage) server, supporting: CIFS (samba), FTP, NFS, AFP, RSYNC, iSCSI protocols, S.M.A.R.T., local user authentication, Software RAID (0,1,5) with a Full WEB configuration interface. FreeNAS takes less than 32MB once installed on Compact Flash, hard drive or USB key.
The minimal FreeBSD distribution, Web interface, PHP scripts and documentation are based on M0n0wall.