Lenovo System x3550 M4 Installation And Service Manual page 316

Step 16. Loosen the two thumbscrews (one is near PCI slot 2 and one is between fans 4 and 5).
Figure 237. Thumbscrews disengagement
Step 17. Grasp the system board handles and slide the system board toward the front of the server until it
stops.
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System x3550 M4 Installation and Service Guide