Hello,   I'm about 99% sure those are not EQL qualified drives based on the symptoms you describe.  Additionally, Dell EMC doesn't sell array drives on arrays that are out-of-warranty.  So I suspect you have  Dell server or other non-EQL drives.  You can either try to get array under contract, which would also allow you to upgrade the array firmware.   6.0.x is years old.    You can also ask them about Dell Extended Maintenance or otherwise try to find a 3rd party maintainer for proper EQL compatible drives.  Regards,  Don   
Does the Dell Equalogic PS4110 support 4kn drives?
PS4110 and 4kn drives