Because you want to use SFTP, see the following posts/threads that may have relevant information. Also do a forum search for SFTP for additional posts/threads. Certain directions are for the v4.x firmware My Clouds.
http://community.wd.com/t/sftp-setup/95350/4
http://community.wd.com/t/how-to-setup-sftp-on-my-cloud/152297
http://community.wd.com/t/ftp-dynamic-ip-a-lot-of-problems-with-config-ftp/172868
