Additional Drive Installation And Casing
If you want to add that additional storage, you'll have to remove a couple screws from the bottom, or well, all four if you plan to get inside. Once all four are removed, you can pull the top off fairly easily, and you'll find that additional drive mount once you turn it over. The mount attaches to the shell of the case via rubber anti-vibration mounts in order to minimize noise, though you can avoid that specific problem altogether by using an SSD instead of a mechanical drive.
All is accessible with just the bottom removed, so taking everything out of the casing and removing the back panel is not required. You'll have to remove two screws to get the board itself out, though.
The back panel of the Beebox is plastic and clips off without much effort. Make sure to remove the cables from the Wi-Fi card as they attach to the top of the BeeBox so as to get the best reception possible. The top also acts as the unit's primary way to rid itself of heat.
The board itself is pretty simple, SO-DIMMs and mSATA drive removed in the picture above. The added drive connector plugs in near the SO-DIMM slots.