Besides the factory 1TB HD, I recently installed three additional hard drives in my MacPro5,1. The same drive has unmounted three times in the past few days. Restarting the system does not re-mount it, I have to open the case and remove the drive then put it back in. At first I thought the issue was with the screws that attach the drive to the tray, the two rear screws did not screw in all the way, they were bottoming out. The drive was a bit loss so I figured the connection was weak. I fixed that yesterday by using two slightly smaller screws, now it's securely attached to the tray and level. I just remoted into the computer and noticed I got the "not ejected properly" message again. I ran a Apple Hardware test yesterday and it came back fine, although the test only took about 2 minutes. I believe there was a check box to do a longer text but my Magic Mouse did not work in this screen. I have not tired swapping the drive around to see if the problem follows the drive or if the problem is to a specific sata connection. I will try this today. The drive was originally in a USB external enclosure and never gave me any issues. Any ideas or tips would be appreciated!