Dell PowerVault ML6000 - Manual (Page 313)

Chapter 12 Installing, Removing, and Replacing
Replacing a 9U Library Expansion Module
Dell PowerVault ML6000 User’s Guide 313
3 Open the 9U Library Expansion Module’s access door and raise the
guide pin by pulling it up and turning it slightly as if it were a screw.
Otherwise, the guide pin may scratch the front doors of the module
on which you are stacking it.