USB3 HDs fail to mount on wake from sleep with new iMac Pro
I had this problem with a WD 6TB external HDD on wake(encrypted HDD) starting with Sierra and continuing into High Sierra. It would show in Finder (but not libraries or files) and DiskUtility, but would not mount. Running FirstAid failed on the physical disk. All was normal on restart. 

Unfortunately It was not seen by TimeMachine and so I ended up with backups without the drive contents. Subsequent backups after restarts ended up erasing all old backups and backing up the entire drive for the next 14 hours.

Uninstalling the WD Utilities, especially WD Utility Helper which runs in background solved the issue. Installing or even just running the WD Utilities executes the helper as well as puts it into the user's startup jobs.