HP RP7 Model 7800 Hardware Reference Manual page 24

The procedure for installing an integrated USB module is the same for all modules. To install a USB
module:
1. Turn off the computer properly through the operating system, then turn off any external devices.
2. Disconnect the power cord from the power outlet.
CAUTION:
as long as the system is plugged into an active AC outlet. You must disconnect the power cord
to avoid damage to the internal components of the computer.
3. Slide down the two levers on the upper corners of the rear I/O panel (1) and rotate the cover off
(2).
18 Chapter 2 Hardware Upgrades
Regardless of the power-on state, voltage is always present on the system board