Yep, appears to be another example of WD Sync sucking up the My Cloud hard drive space again.
If your using Windows, you may want to use WinSCP (https://winscp.net/eng/download.php) as its a bit more user friendly when accessing the My Cloud via SSH. You may have to set WinSCP to show hidden files through to see any files in those WDSyncHistory folders. See the following link for more information on enabling hidden files option: https://winscp.net/eng/docs/ui_pref_panels
Edit to add: To stop the issue in the future, don’t use WD Sync if your syncing on the local network. Use a third party sync program like Free File Sync (http://www.freefilesync.org/). Just be aware that you cannot remote sync with most third party software except in certain cases like over FTP (or SFTP). Further note that FTP is not a secure protocol.
