MyBook Thunderbolt Duo powering itself off

Hello, all,
I’ve had a MyBook Thunderbolt Duo for years and it’s been working fine. In the last three weeks, it turns itself off and then back on in the middle of the night. I put my iMac to sleep at night, and when I wake up in the morning, I get the messages from macOS that the disks (there are three partitions) were not ejected properly. One morning the drive hadn’t recovered yet, but the front light was on. I had to unplug it and plug it back in again. The drive is connected to a UPS, and all other devices on the UPS don’t shut off. The UPS is brand new. Any ideas?

EDIT: I’ve been running Mojave since it was released, so there are no major upgrades to macOS, or any other changes. Also, there was no WD software installed on the Mac since I installed Mojave.

I upgraded my iMac Pro to Bigsur 11.1 and when I turn on WD 8TB Thunderbold DUO external HD it keeps the computer shut down