I am a new user of WD Smartware and would like to know if I perform a backup of my hard drive will the files that are already on my WD drive be deleted or replaced?
No, Smartware will only add files. The only time when anything gets deleted in the backup is if you set a maximum number of versions of the same file to keep, then if you go over that the oldest version will be removed.
If you delete the file on your PC then it will remain on your backup drive. (unless you manually remove it). But it stops too many versions building up and filling your hard drive.
Smartware keeps its backup firstly by computer (as it can be used across the network to back up multiple machines), then below that it will have individual folders per hard drive on each PC, and then within that the structure on the drive will be replicated as per the backup settings.