Hey Donncha, why DVDs? the price per GB of HDD is competitive, especially considering that most DVDs sold today are of cheap quality – no good for archival. My experience with DVD backups is that they deteriorate rapidly (had read error after two years on “good” brand DVD), they need to be kept in absolute darkness. I always kept double-copies and checked them (checksums) every six months, replicating on new media when one of the two copies had an error. I stopped. This kind of manual RAID is time consuming, environmentally unfriendly and expensive. I now use offline SATA HDD. On my desktop machine I have a panel with a SATA connector. Plug it in, back it up, and store away in a foam padded pelican case (the main risk is mechanical shock). My photos and videos are currently growing at 10 GB per week. They first go on a 2TB network attached RAID, and once in a while I take the archive HDD out of the case and rsync to it.
Happy New Year