A common problem when you have a lot of big files with pictures taken with you camera is that those has a certain value and it is good to have one ore many backups. A PC is heavy and a hard drive isn't directly compatible. A raspberry Pi may work with all USB interfaced cameras, in my case a couple of Nikons to become a HD-drive in a Linux system.
Therefore, i suggest a Pi (supplied on internal battery) with a portable disc of some kind with a display and buttons for the control, but it need to be a very compact design. I don't know if it should contain the drive in an enclosure or if it should be possible to swap?
Btw, this isn't even close but is expensive: http://www.bhphotovideo.com/c/product/1191265-REG/sanho_shdcsudma3500_500gb_colorspace_udma_3.html