Running out of space while using WD Sync is a well discussed issue in this subforum, see past discussion on this at the end of this post.
One solution is to stop using the WD Sync program until WD gets around to addressing this problem and use a third party sync software. Note that using third party sync software may not allow for remote syncing using methods other than SFTP or FTP. Free File Sync (http://www.freefilesync.org/) is one such third party program some here have resorted to using. If installing Free File Sync make sure NOT to install the adware (OpenCandy stuff) that if offers up (usually on the fourth installer screen).
A few of the various other past discussions on this particular problem:
https://community.wd.com/t/wd-temp-file-on-c-drive-filling-up-hard-drive/175208
https://community.wd.com/t/wd-hidden-files-directories/168494
https://community.wd.com/t/wdtemp-folder/145676
https://community.wd.com/t/dashboard-says-im-out-of-space-but-im-not/172540
https://community.wd.com/t/my-cloud-file-filled-up-c-drive-should-i-delete/160417
https://community.wd.com/t/warning-using-my-sync-for-my-cloud/152857
