it is stored on all the drives. It manages all the data across all the drives. You can 4 different size drives or all the same. It dedicates the largest of the drives to helping span the data. They have a decent video tutorial on the front of there website that shows how it all works. I will be Building another Drobo that mirrors this one and will be keeping it in another location. Plus My whole house is on UPS's so I feel a little better about surges etc. All in all it manages the data for me and i dont have to try to manually syncronize multiple drives all the time and spend house making sure it all copied ok with no errors etc.
The unique part of it is it is not raid, it is its own technology. So you dont have to put the drives back in order if you ever have to replace the unit, it just knows whats what when you put them back in. You can use drives of different size, and it makes use of the capacity unlike raid etc.
Thanks RichNY, I have been working with it all day getting it setup etc.
I am actually going to 'put it to sleep' when not in use backing up photos that way it is not on all the time and unplugged from the wall untill I need it again.
Gear List? My gear is bigger than yours? Just shoot have fun...