<< But then BB greys the bars 3&4, and pastes the bars 1&2 into the bars 5&6, which become bars 3&4. What am I missing? >>

Looking at your screenshot, your issue is the blue part marker in Bar 3. You correctly copied and pasted the bars and the copy/paste included the Blue Part Marker.

Simply click twice on the Blue Part Marker in Bar 3 to remove that Part Marker and two grey bars.

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BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.