Funky has the only correct answer.
Back up to CD[700MB]/DVD[4.2GB]/Blu-Ray[25GB].
This will require the appropriate optical drive and suitable media. Drives can be either mounted internally or externally [via USB2 or FireWire]
No other option has so few risks.
Hard drives ALL fail...not if...when.
Additional hard drives are great for a growing collection, but not for archives.