Secure Copy (SCP) is a protocol for transferring files between two computers over SSH. When you have enabled SSH on your MyBook World Edition, using SCP is a breeze. It offers largely the same functionality as FTP, but FTP is insecure: passwords and files are sent over plain text, making it easier for third parties to access your data.
Just enable SSH on your MBWE and use an SCP client. Once SSH is enabled on the MyBook, no further configuration is needed to access it using an SCP client. On Unix-like systems, the command line client is aptly named 'scp'. Popular GUI clients are Winscp for Windows and Cyberduck for Mac OS X.
The problem is, SCP is like SSH and SSH has access to *everything* on your MBWE. Especially running as root is a bad idea from a security perspective. SCP does not have a text-file based configuration, access restrictions can be set with 'chmod' and 'chown'.
Users have, by default, access to all files you'd normally see as shares: they can be found under /shares/internal and /shares/external. It is a nice idea making a symlink between your user directory (under which you start your SCP session) and the /shares/internal directory. This can be done when you are logged in under SSH as the user you want to use SCP with:
cd / ln -s /shares/internal ~
Another potential concern using SCP on the WD Mybook is that since all traffic is encrypted, the CPU must spend effort encrypting or de-encrypting the files that go through. On modern desktop or server CPUs, this overhead is insignificant. For the MyBook's ARM processor, this overhead carries a noticable performance penalty, so file copies over SCP are much slower than they are over, say Samba (Windows shares) or FTP. If you are accessing your MyBook over the internet, the performance penalty may be worth the added security. If you are transferring large files on a local network, using some other protocol may be better suited.