For price per Gigabyte, DVD is by far the cheapest medium to store data on, but as you mention, if you use a large number of discs, there is a very good chance of having one or more unreadable discs later on. If you plan backing up a large amount of data (e.g. >100GB), DVD is also the most tedious (after CD), as you need to change disc after each one is filled.
Unless money is not an issue, I would also suggest avoiding Blu-ray, which in my opinion is a rip-off compared to other storage mediums, including hard disks! Like CD & DVD, there is also the risk of a disc becoming unreadable later on. The main advantage over DVD is that you don’t need as many discs when backing up a large quantity of data, however, if you plan backing up over 500GB worth, even this storage medium can be quite tedious.
For convenience and the next cheapest after DVD, hard disks have the huge advantage of storage capacity. For up to the hard drive capacity, you can backup everything in one go, leaving the backup process run overnight. Use a 2nd hard disk if you really want to make sure the data can be recovered. The chances of losing files on your main hard disk along with two backup hard disks failing together is extremely rare. Just be sure to keep one off-site.
A few tips depending on the amount you want to backup:
[li]<5GB - Write this to 2 DVDs or several CDs or to a a combination of CD/DVD and online storage[/li][li]5GB to 50GB - Make 2 sets of DVDs from this or a set of DVDs and to removable media (e.g. ext. HDD or flash drives)[/li][li]50GB to 500GB - Store the data on 2 external hard disks. If you have Blu-ray or tape drive, you can use a single hard disk and make a set of BD-Rs or tapes as the 2nd backup.[/li][li]500GB+ - Store the data on at least 2 external hard disks.[/li][/ul]
Follow midders’ advice also. As midders mentioned, it is very important to keep at least one copy off-site. E.g. Even 1,000 back-up copies of that precious once in a lifetime photo is no good if they are all destroyed in a fire.
The following gives an idea of the price to back up 1TB on two sets of same media, assuming you have the necessary drive (DVD or Blu-ray writer) if required. These prices are based on Dabs Ireland, so the pricing will likely differ to your area:
[li]4 packs of 100-spindle of Verbatim DVD+R 16x (2 sets of ~940GB) - â‚¬100 (10.6c/GB)[/li][li]2 Western Digital 1TB Elements Black USB drives - â‚¬167.62 (16.8c/GB)[/li][li]8 packs of 10-spindle of Verbatim BD-R 4x - â‚¬485.68 (48.5c / GB)[/li][/ul]
Tape storage has a high initial cost for the drive, but if you plan backing up several Terabytes worth of data, tape storage is very reliable from my experience of working with servers. Unlike hard disks and other media, it is very rare for a tape to become totally unreadable, unless of course it is lost or is destroyed. On the other hand, recovering individual files can be quite tedious, as like with any other tape medium, it takes a while for the drive to seek each file. A full recovery on the other hand goes relatively quick, as the tape is simply sequentially read. I would suggest avoiding the tape route unless the content you are backing up is business related.