Also one other note - if you have a few faces selected out of an object instead of the whole object, when you do Separate there, it will break the selected faces off from the main object but still have them connected between themselves. That allows for kind of extracting a sub-assembly without having to re-join it.

If you want to break a face sub-selection like that into individual surfaces, hit separate a second time which will then break it into individual surfaces.

If you have a "whole object" selected, it will break down into individual surfaces when you do separate. If you have a face sub-selection then will do the "extract face sub-assembly" type thing.

You may have run into this - if you had a face sub-selection and did separate you did not quite get things down into individual surfaces at that point yet.

- Michael