Lenovo ThinkPad T520 User Manual page 202

Note: If you attach your computer to the ThinkPad Mini Dock Plus Series 3 or the ThinkPad Mini Dock Plus
Series 3 (170 W) and use an external monitor connector, use the external monitor connector on the ThinkPad
Mini Dock Plus Series 3 or the ThinkPad Mini Dock Plus Series 3 (170 W), not the one on the computer.
10 When you attach a mechanical lock to the security keyhole, the eject button is locked so that the
computer cannot be removed from the ThinkPad Mini Dock Plus Series 3 or the ThinkPad Mini Dock Plus
Series 3 (170 W).
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User Guide