At the OS level, which is Debian Lenny Linux, you have a few options.
If security is a requirement: Rsync
If not, FTP.
Plenty of info out there on the internet
Some links:
http://www.thegeekstuff.com/2010/09/rsync-command-examples/
http://www.mikerubel.org/computers/rsync_snapshots/
http://community.wd.com/t5/My-Book-Live/GUIDE-Enable-FXP-transfers-between-2-MyBookLives/td-p/535436