I just purchased a refurbished HP Compaq 6000 pro SFF and ran into the same issue trying to remove the access panel. When I pulled on the latch, the back part of the chassis with the I/O ports (and plenty of holes for ventilation) would simply flex inward.
Here's what I did to open my HP Compaq 6000 pro SFF:
1. To bring the part of the chassis that was flexing inward closer to the access panel, I inserted a screwdriver into the padlock ring.
2. Keeping the screwdriver in the padlock ring, I used another screwdriver to put a screw into the screw hole in the access panel just behind the latch.
3. The back part of the chassis that was flexing and the access panel were now close enough for the latch to work, and the latched snapped open when I pulled on it fully.
4. While still holding the latch, I removed the screw from the screw hole and the screwdriver from the pad lock ring, then I lifted up on the access panel and voila, it opened.
Hope this helps!
