External hard drives expand storage capacity, share data between computers, and backup files. There are several ways to use an external hard drive to keep a backup of the data on your Mac in case the built-in storage fails. You can create a full backup of your entire hard drive, including the operating system, or simply backup certain important files in case they are lost from your Mac.
Carbon Copy Cloner
Download Carbon Copy Cloner (CCC).
Double-click the DMG file you downloaded to mount the CCC disk image. In the window that appears, drag the CCC icon to the "Applications" folder alias to copy CCC to your Applications folder.
Launch CCC from your Applications folder. The first time you launch the program, click the "Agree" button in the drop-down pane to acknowledge the license agreement, then click "No Thanks" to proceed to the main interface.
Select your Mac's built-in hard drive from the "Source Disk" drop-down menu, then select your external hard drive from the "Target Disk" drop-down menu. If your external hard drive is not in the list, power it on and connect it to your Mac, making sure its icon appears on your desktop. Leave the "Backup everything" option selected in the "Cloning options:" drop-down menu, unless you only wish to back up some of your Mac's data. You can also choose "Incremental backup of selected items", then specify which files to copy using the checkboxes in the file list that appears in the left pane of the window. Note that the external hard drive must be formatted using the HFS+ file system to create a backup with CCC (see References 1).
Click "Clone" to begin the process. Then Enter your administrator password when prompted to do so, and wait until the process completes. Be sure that you have rewiewed the backup scheme you've specified in the "What is going to happen:" field near the bottom of the window.
Power on and connect your external hard drive to your Mac. A dialog will appear if you aren't already using Time Machine or haven't yet dismissed it when using this hard drive. Click "Use as Backup Disk". The System Preferences program will appear, with the Time Machine pane revealed to show your backup settings. The backup process begins immediately. Once the initial backup is completed, the backup will be updated every hour to reflect the changes you make on your Mac.
Launch the System Preferences program from the Apple menu if you did not receive the Time Machine dialog automatically upon connecting your external hard drive. Choose the "Time Machine" pane from the "View" menu to reveal the Time Machine settings, then click "Select Backup Disk" and choose your external hard drive from the dialog that appears to initiate your backup. Note that the external hard drive may not be formatted with a Windows file system if it is to be used with Time Machine.
Click the "Options" button, then use the button with the plus symbol to select and add files and folders to the "Do not back up" list. Click "Done" to apply your changes.
Click the Time Machine icon in your dock to access the data in your backup. A timeline appears on the right side of your screen, and shadowy copies of any open folders will appear receding into a picture of a black hole. You can move through the timeline using the two arrow buttons near the bottom right corner of the screen. Open a Finder window and select the file or files you wish to retrieve from the backup, then click the "Restore" button.
Conduct a full system restore from a backup made with Time Machine by booting your Mac using the OS X installation disk. In the installer interface that appears, choose "Restore System from Backup" from the "Utilities" menu, then click "Continue" and follow the instructions, selecting your external hard drive when prompted.
Power on and connect your external hard drive to your Mac. Wait until its icon appears on your desktop.
Double-click the external hard drive's icon on your desktop, then press Command-Shift-N to create a new folder in the hard drive to store your backup. Create as many folders as you wish to organize the data you want to back up.
Double-click the icon of your Mac's built-in hard drive (named "Macintosh HD" by default) to reveal its contents. Navigate through the file system to find the files and folders you want to back up, and drag each to the backup locations you created on your external hard drive to copy the data. Dialogs will appear to show the progress of each copy. When the copying is complete, your files are stored on the external hard drive and can be accessed at a later date and restored by dragging them back to the built-in drive.
In the Carbon Copy Cloner, the "Backup everything" option is designed to copy all of the files from your Mac, including the operating system itself, so make sure your external hard drive has at least the storage capacity of your built-in hard drive if you want to create a complete backup. If you've created a full backup by using the "Backup everything" option, the external hard drive now contains all the data necessary to fully restore your Mac to its operating condition at the time you made the backup. You can even boot your computer directly from the external hard drive using this backup. You can restore your Mac using this backup by following the process described here, but selecting your external hard drive as the source and your Mac's built-in drive as the target.