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I unchecked the box for 'Put hard disks to sleep when possible' in the energy saver panel on system preferences and it appears to be fixed. Hallelujah!
 
I unchecked the box for 'Put hard disks to sleep when possible' in the energy saver panel on system preferences and it appears to be fixed. Hallelujah!

I'm having this problem with my brand new WD 4TB passport (I had it backing up the drive and when I woke up, I saw that not ejected properly message even though it was plugged directly into the Mac Mini). It might be that sleep setting because WD Drives typically have their own firmware sleep setting that ignores the Mac sleep message or confuses it. I've got a Seagate 8TB drive and it's just the opposite. It won't sleep at all unless you check that box in preferences. I actually got the 4TB smaller 2.5" drive because the 8TB one takes too long to spin up (Kodi on FireTV units in the other rooms of the house will complain that it can't find any shares or the file is missing when the 8TB server drive is waking from sleep yet I don't want it spinning all the time as it would wear out prematurely and it doesn't get typically used 16 hours of the day or more. So I figure two small drives would work better than one large drive for serving media. Once everything is done copying, the 8TB one will be used as a multi-partition backup instead so I guess I can then turn off the sleep setting. I just hope that helps and it's not a bad drive.
 
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