I also can not understand how it can be possible until yesterday.
I use a Mac mini with single drive at my work and I have a macbook with double drive at home.
I was in a hurry and connected my external drive to macbook pro for wipe all data and change FS. So I opened disk utility and I did not pay attention to that macbook did not recognized external drive and click on last bottom drive (that was my second inner drive) and click to change FS...that`s it.
So, I tried a lot off apps and Data rescue 4 is amazing. It restore 100% my data (for the first look) with names and structure. I`m really happy!