MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Leaving a single layer gap is how rafts are normally done. You do get some adhesion between the 4th and 6th layers, but is should be weak enough that it can be broken free pretty easily. (There's not that much surface area on the bottom of the outer sphere.) If it is really firmly stuck, you may be over extruding or printing at too hot a temperature or printing too slowly.