Your best bet too is not to do it under the subdiv attach anyway, its a little flaky, it always has been right back to the days of Maya 3. god I feel old!!

Your best bet is to switch back to poly mode, switch on your border edges to see the unattached edges (they are thicker) and then use combine to get the two halves together, followed by merge vertices on a low tolerance so you dont pick up any other verts that dont belong to the ones being merged. Once this is done then convert back to a subd. Also check for flipped normals on the duped half before combining too, just save a lot of hassle later as well.

I used to use subds religiously for years until the advent of smooth poly (keybard 3) its just so much easier to jump back and forth between the low res and a smooth version without the ballache of subd nodes and blind data getting in the way.

best
Jay