Hi all, I own a mac pro 3.1 running OS X 10.11.3 with 4 drive bays and already am using a Seagate ST3000DM001-1CH166 in one of the bays without any problems at all. Last week i bought two new drives which are basically the same drive and 3tb capacity but they have a slightly different part number : the two new ones i bought have pn: ST3000DM001- 1ER166-501. These two drives are not playing well at all in the mac pro. The problem isnt helped because in 10.11 disk utility has been so badly dumbed down but on one drive i am unable to unmount, format to either hfs+ or fat and then mount at all, disk utility fails, and the other I can format to hfs+ but the drive is incredibly slow and seems unstable. The computer will not open quickly with it installed as if it is having great difficulty to be read and hangs on startup for a few seconds as finder tries to work out what is going on. Write speeds on that drive are in the order of 1 minute for 100mb of files, very slow indeed. I am guessing that there may be a problem with the firmware of the newer drives being incompatible with the mac since it is so hard for me to mount and format the drives, and since the other similar drive works fine. Has anyone had a similar experience or have any good ideas? All contributions gratefully received!