For whatever reason Apple has a penchant for using different screw head types to hold the same assembly in place. For example the logic board in some Mac mini models is secured by four torx screws and one hex screw. OWC's disassembly instructions carefully warn that although a T8 driver will work on all five there is a definite risk of stripping the hex screw unless you are very careful. I am wondering if that is the case here? A screw head for a small hex driver can be deceiving to the eye.
