I'll try and explain as simply and succinctly exactly what it is I require.
It's not for me but a friend, but let's say it were me....

I have a 'Pictures' folder and inside there many folders and sub folders.
I make additions or edits to these

But I'd like to keep a backup. That can easily be managed.

*The cloud is a possible but currently the collection is 50GB

The friend is actually using Mint 13 (AKA ubuntu 12.04) and uses Picasa because it organises the images in some way that is required.

But could we have a external HDD and keep Pictures from the PC mirrored to the external, in an uncomplicated manner?

Any suggestions welcomed