Yes Robbie, but what if that one external hard drive fails? That could mean disaster.
You need to have a copy of those pictures in 2 places mate. ONE external hard drive is a single point of faillure

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If it is just for storage, just hop in the computer shop around the corner and buy yourself a good USB drive or if speed really matters go for an E-sata model. And if you can't decide between speed or comfort, just buy one that can do both:d