Searching for "disjoint features", it seems a number of people have had this problem before.  I am trying model a slab with a cut-out each side. When I try to mirror the top cut-out in the slab's mid-plane, I get the familiar message "Some feature instances are disjoint and will not be created. Use the bodies option to mirror/pattern discrete bodies".

I have two issues with this.  (1) What I am trying to do is very straightforward - surely SW should allow it?  It clearly does not create a "disjoint feature".  What chance have I of completing the complicated final design when it falls over at the simplest stage?

(2) The feature I am trying to mirror is an extruded cut. When creating the cut, I don't see any option to define it as a body so that I can "Use the bodies option".

The problem may relate to the way I have defined the cut.  The sketch is drawn on the mid-plane, but the cut runs down from the top plane.  Again, SW should be able to handle this.

For reference, the final design will look like this, with the underside a mirror-image (mostly) of the top.

I am trying to define one quadrant and mirror it downwards, backwards and sideways to get the 8 copies.  I then want to create lids for the top and bottom that match the outer profile, bolt line, troughs etc but I am struggling to find a way of transferring the sketches for these to another part, whilst allowing me to continue to tweak the overall design.  Would I be better off making it as a multi-body part based on a single sketch (on the mid-plane, say), or perhaps saving a single, solid object and then using a series of extruded cuts to just leave a top lid/centre slab/bottom lid?  This must be a common requirement but I haven't found any tutorials on how to do it.

Thanks

Roger