With x2 500GB Disk (Mirror) That means you have Raid 0 So the way you do this is complex and specific. You Remove each drive one at a time and mark which one is Drive 0 and Drive 1 on the drive with a sharpie and label what slot it goes in. You then put 2 NEW 1TB Drives in and Format and Raid them exactly the same as the old pair FROM THE BIOS RAID MENU. You do this using the bios menu so that you have a fresh Raid 0 on the new 1TB drives. You then shutdown and restart to find out if the raid controller thinks everything is ok with the raid 0 container. Shutdown again and remove each 1TB drive one at a time making sure to mark which one is Drive 0 vs Drive 1 with a sharpie and label what slot it goes in. You now put the old 500gb pair back in and restart and make sure the raid says everything is ok. Once it restarts a few times to verify its ok shut down again. After Shutting down with power off remove the 500gb DRIVE 1 and put in the 1TB DRIVE1 in its place. Turn on and go into raid menu immediately and rebuild the mirror from Drive 0 to Drive1. DO THIS FROM BIOS NOT FROM THE OS. Once its rebuilt reboot and it should still think you have 2 drives 500gig mirrored. Restart a few times to verify the raid is happy. Shut down again. Now remove Drive 0  500 gig and replace with the 1TB Drive 0 Turn on and IMMEDIATELY go into BIOS Raid Menu and Rebuild Drive 0 from the new Drive 1 in the BIOS MENU Once this is done and the raid controller is happy you can try to restart and see if the os knows about your drives. It will still show it has 500gigs of space but will have an additional 500 gig unused. However with the 1TB drives installed and formatted by the raid controller the OS will now know that there is un used space on the new 500 drives.  You can use DISKMGMT.MSC to expand the partition on drive 0 to the new 1TB space. The reason to do it this way is that you can always go back and remove the new drives and put the old drives back and you have not lost any data.    
Good day Dell Com -I have Dell PowerEdge T320: Running Win Server 2012 R2 -With x2 500GB Disk (Mirror) -I am running out of space and what to add x2 1TB to the existing setup -Kindly share with me the procedure on how to do it.  NB: I have never configured RAID and I dont want to mess up with the existing data
Increasing Disk Space