Why does my Time Machine external drive unmount overnight after upgrade to Mojave?

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 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.