If you want complete saftey then you would need a multiple approach. Firstly Something Like Raid-5 which basically makes 2 copies of the same file on a hard disk array, this would protect you from disk failure. You then need a backup tape/media which you do an incremental backup of the media on a daily basis, this would store any new files or files changed that day, you then need a weekly backup which would be a copy of all the data, these tapes would be stored in a fire proof safe or for total security at a different location miles away from the location of your servers.