I have just purchased 2 HGST 4 TB hard drives to increase my storage options. I have a 2010 5,1 Mac Pro running 10.10.5 Yosemite. I tried initialising mac OS Extended(journaled) on the first drive in bay 1 using disc utility. After one failed attempt it formatted after a second time and I thought all was fine. However the drive is only formatted to 400.09 gb and despite trying to re initialise it is stuck on that figure. The second drive was initialised successfully in an external drive reader, and using drive genius this time, GUID partition table checked. When mounted back in the Mac Pro it reads as a 4TB drive. Using the external drive reader and drive genius and disc utility I cannot re initialise the 1st drive to any thing other than the 400.09gb. Any ideas on what I can do to re-format this drive correctly to 4TB , assuming there is nothing wrong with the hardware?