Whats the slot in the back of my Mac with the chain icon?
That’s for a security device. Several third-party manufacturers made a small metal slab with a ring on it that snapped into the slot and was impossible to remove without opening the case up. The ring was for running a security cable through in order to lock the Mac to a desk, table, or other secure surface. If you have the security rings in several Macs and want to remove them, simply push the device firmly into the interior of the case (this may require the case to be opened in some instances) and it will pop out.
