This is the same solution that I bought back in 2005 .. a pair (yes, for redundency) of Sanho Hyperdrive HD80's. At the time, they cost around $250each for 40GB of backup storage capacity. I've modernized them once, to use 120GB internal PATA 2.5"HDDs. The stuff they're selling today is probably better several ways.
Originally Posted by VirtualRain
Honestly, the best solution, is just to keep your photos on the SD card...
...and if you think that 32GB isn't enough, just buy some more memory cards - by today's standards, they're reasonably affordable and they're small ... even CF sized cards:
..that's 200GB, which IIRC is around 4500 images on a Canon 7D (RAW+JPEG).