After High Sierra Upgrade, my usb drives disconnect at night

After High Sierra Upgrade (10.13.1) from Sierra on my iMac (late 2013 version), my usb drives used for TimeMachine become disconnected / inaccessible at night and TimeMachine backups fail. . The drives are Western Digital My Passport for Mac 2T drives. In the morning, I unplug the drives, plug them back in, and everything works again, and TimeMachine starts a new backup. This never happened before the upgrade, and started happening the 1st night after the upgrade. Ran disk utility and the WD drive tool, and both say the disks are fine. So, has to be related to the upgrade.

I think this is related to Energy Saver Sleeping. In Energy Saver settings, I have checked

- Put hard disks to sleep when possible

- Wake for network access

- Enable Power Nap

Schedule is set to "Sleep everyday at 1am."

All the TimeMachine failures happen after 1am. Last backup is around midnight and then nothing until I reconnect the drives. I will try turning off the "Sleep everyday at 1am" to see if that removes the issue, but this was never a problem before. Seems like a bug was introduced in High Sierra in Sleep in handling USB drives.