Solution #1 Launch the WD App Manager manually after every reboot of macOS 10.13 computer. Manually start the WD App Manager in Applications - WD App Manager.app.
To check whether the operation is really successful, go to your Mac desktop. You should see multiple disk icons show up. I chose to create two partitions on my Seagate Expansion — one for backup, the other for personal use. You can find more info in this post: How to Backup Mac to an External Hard Drive.
WD Sync will start processing data Solution #2 Uninstall, Install and Setup WD Sync after upgrading to macOS 10.13 (High Sierra). Uninstall WD Sync.
For assistance, please refer to. Install WD Sync. For assistance, please refer to.
Configure WD Sync. For assistance, please refer to Important: Adding an additonal or new WD Sync Folder is not supported on macOS (10.13) High Sierra at this time.