The easiest way to make backups of a Mac is through Time Machine > http://support.apple.com/kb/HT1427
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.
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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
Yes, you can back up as many computers as you want on a single My Passport. It will automatically recognize the correct computer when you plug it in. You can also use up to 3 Passports on a sigle computer. I do this so one is backing up, one is safe at home, and one is in transit between work and home or home and work. Format using disk utility on any Mac such as: Macbook Pro, iMac, macbook air, laptop mac, desktop, computer apple, macbook retina display, macbook air Disk Utility can also combine multiple hard disks into a RAID set that acts as a single disk. Using multiple hard disks in a RAID set can increase.