Lenovo ThinkStation P520 Hardware Maintenance Manual page 164

a. Lift the plastic PCIe card latch at the rear of the computer. Then, pivot the latch outward to rest it on
the edge of the rear panel.
Figure 108. Opening the PCIe card latch
b. If the PCIe card is connected to another device, disconnect the cable from the card first.
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P520 Hardware Maintenance Manual