Where did you get the drive? I suspect is in not an actual Dell Equallogic drive. You cannot use OEM or other Dell branded drives that are not specifically made for the Dell PS Series SAN. For example a PowerEdge server drive will not work. Even if had the same drive part number.
The 12Gb drives sync down to 6Gb so that is not an issue.
What revision of firmware are you running? If it's very old that too can cause compatibility issues.
I can't really tell from the photo. But 6100/4100 use the same drives. So likely it is not a true EQL drive. It might from a powervault array for example.