The easiest way to make backups of a Mac is through Time Machine > http://support.apple.com/kb/HT1427
The second version the WD My Passport comes in is a ‘for Mac’ version. This means it is preformatted at the factory to HFS (Hierarchical File System) WD My Passport for Mac version. It is worth bearing in mind that HFS and Mac OS Extended (Journaled) file system are one and the same.
To use Time Machine, open System Preferences > Time Machine > Select Disk, and choose your external drive. The initial backup will start, and it may take several hours, depending on the amount of data Time Machine has to copy to your external drive.
Since then, Time Machine will make automatic, hourly backups, always that your Mac is turned on and the external drive connected. Also, if you are using OS X Lion, local snapshots will be enabled > http://pondini.org/TM/30.html