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I use a LaCie Quadra RAID drive as my Time Machine drive. It's directly connected to the USB socket on my iMac (Retina 5K, 27-inch, Late 2014, macOS 10.14.1). It's been fine for years.

It may be a coincidence, but I upgraded to Mojave recently, and the LaCie drive started sometimes unmounting overnight, when the iMac is sleeping. It doesn't do it every night. All I get from the iMac is a message in the morning telling me to eject drives before unplugging. The RAID drive is still properly mirrored when I power cycle it to remount. I use the iMac almost all the time during the day and the drive has never unmounted in the daytime.

How can I find out more about what is going on? I'd like to understand whether my drive controller is breaking or some software has started idling my USB or...

Thanks!

  • I sometimes have this with the CF card in my 2015 Retina MacBook with Sierra and High Sierra. Never could find the cause. – Jeroen Wiert Pluimers Dec 2 '18 at 6:09
  • You state that "All I get from the iMac is a message in the morning telling me to eject drives before unplugging", but I see no verification that drives actually have been unmounted. I've had this notification issue for a while, not only about disks. When I changed the time for enabling "Do Not Disturb" in Notifications from 11.59PM to something else this morning, it seems this have stopped. But I cannot be sure until some more time has passed as this could be about some kind of queue build-up. – MiB Dec 13 '18 at 10:35
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I did eventually find out why my drive unmounted. It was nothing to do with Mojave. The drive was breaking. I had to replace the drive, and the new one has had no problems.

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