Your chart implies that CD and DVD have the same standard 1x speed - they don't. A DVD's 1x speed is about equal to a 20x CD (http://www.computeractive.co.uk/features/76887).Without this information, the data is a bit misleading with regard to optical storage.
Also, as a disadvantage to the USB and Firewire hard drives, I would have listed the potential of drive failure due to mishandling of the device.
Total cost per MB of storage capacity could help drive decisions along with mention of off site storage of a set of backups. This would make the (up front) cost of external hard drive backups pretty high compared the optical alternative.
Just my $0.02 worth.
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