Dell Alienware M11x R2 (Mid 2010) - Manual (Page 65)

CHAPTER 4: INSTALLING AND REPLACING COMPONENTS
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To replace the memory modules, perform the removal steps in reverse order. While inserting
the memory module into the connector align the notch on the memory module with the tab
on the memory module connector.
NOTE: If you need to install memory modules in two connectors, install a memory module
in the lower connector before you install a memory module in the upper connector.
NOTE: If the memory module is not installed properly, the computer may not boot.