On my 2012 Mac Pro 3.06 gig Desktop with 2 Western Digital external drives, the next morning I almost always get the message Drive not properly ejected, and the drive, or drives are missing from the desktop. I have the latest firmware in the drives. all my drives check out fine with Diskwarrior 5.0, and disk Utility. This never happens to my 4 internal drives. What could be the problem? I'm running the latest version of Yosemite.