I'm new to this tool, going through Learning 7 Modeling & animation Handbook.
NURBS tutorial shows that you should be able to split a half sphere along 4 isoparms to get 6 adjacent surfaces. The tutorial, it's demo movie and Maya help show that you should be able to just delete a surface to create an open space.
The result I get is 6 overlaping pieces that are duplicates of the sphere cut along each isoparm and the original sphere is left behind - 2 vertical cut half spheres, a center piece cut vertically, a horizontal piece, 2 horizontal cut half spheres and the original half sphere. I've turned off keep original and saved the setting.
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I'm using Maya Personal learning edition 7.01 upgraded from 7.0 when I started having this problem.