HP rp5800 Reference Manual page 33

7. Rotate the power supply all the way back to access the full-height expansion card retention
latch.
NOTE:
from the rear of the computer so that the connectors do not block the power supply from rotating
all the way back.
Figure 2-23
8. Pull back the green tab on the full-height expansion card retention latch to open the latch.
Figure 2-24
ENWW
If you are using a PS/2 mouse and/or PS/2 keyboard, make sure they are unplugged
Raising the Power Supply
Opening the Full-Height Slot Cover Retention Latch
Installing a Full-Height Expansion Card 27