Are you using WD Sync?
If so, that is typically the cause.
Much past discussion, see some of it below, on WD Sync filling the hard drive and or creating hidden folders/files. Part of the problems are tied to the versioning the WD Sync program does to allow one to restore a previous version of a file or one that was deleted.
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
https://community.wd.com/t/how-can-mycloud-be-full/158401
If not using WD Sync but using Smartware it is possible the versioning option of the Smartware program which typically saves up to five prior versions of a backed up file may be the cause.
