HP Compaq d538 SFF Hardware Reference Manual page 26

Hardware Upgrades
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2-16
8. Install the expansion card.
Installing an Expansion Card
9. Rotate the slot cover retention latch back in place to secure the
expansion card.
When you install an expansion card, make sure to line it up with
the corner bracket, then press firmly on the card so that the whole
connector seats properly in the expansion card slot.
10. Replace the expansion card cage.
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