Well, you've now learned lesson 1.
You'll need an external HD formatted GUID partition table and Mac OS Extended (Journalled) file system (you can do this from DU of course).
You can clone the internal drive to the external using the Restore function in DU.
Select a volume in the left column and open the restore tab.
Drag the Macintosh HD icon onto the 'source' window
Drag the external drive volume icon onto the 'destination window.
Check they're correct, then click 'restore'.
This will take rather a long time, depending how much data is to be transferred.
For future updates to the backup, you should get CCC or SuperDuper. These can do incremental backups, whereas DU is 'all or nothing'.
For an external hard drive, I recommend, and use, drives from OWC. Many of their cases use Oxford chipsets, which work well with macs, and most drives come with a 3 year warranty.
If you want a drive which is portable and doesn't rely on wall power, this is one. We have several of these and they have worked without any issues. If you want one that stays home, this is one. And there are others on the OWC site.
While you can backup your system, erase/format the drive, and restore, some have found that Disk Warrior can repair the hard drive. Of course, since you should have a backup, once you get that a hard drive, a backup/restore can be done. Then you can use Time Machine to do incremental backups, or as noondaywitch said, SuperDuper! and CCC can also do that. I believe for SuperDuper! and CCC you need to purchase them to get incremental backup capabilities. Time Machine is included with OS X 10.6 Snow Leopard.
Hajar Fahad wrote:
how to do back up when I can't open lap...
You can do it like noondaywitch says or you can do a better approach which is to install OS X on a external drive.
Your going to have to erase and rebuild your boot volume anyway.
So if you do this on a blank powered external drive first, then option key boot the computer from it, you can grab just your FILES off the internal drive which has a higher chance of success (and faster) with a problematic drive than copying the whole thing you can't use anyway and incur a higher chance of failure in Disk Utility.
If you can't transfer files normally with drag and drop methods from the external boot drive (like they disappeared from the file system) you can install Data Rescue on the external booted drive and use that to scan the internal drive for files to recover by the 1's and 0's of the files themselves on the magnetic media.
Since the external drive is bootable, you can use the computer, get online and whatever you need just like normal.
Once all your files are safe and the external drive is setup the way you want with programs, tweaks etc., then you can download Carbon Copy Cloner and clone the external drive back onto the internal one using the default settings. The cloning process does all the work.
Disconnect the external drive and reboot, set the internal drive as the Startup Disk in System Preferences.
So with this method you have a greater chance of success recovering files,
The Disk Utility > Restore method is dangerous in the fact that you won't know what DU missed copying until you erased and installed on the original drive, unless you have another Mac laying around that you can verify the files were copied.
Once you erase the internal drive or partition, your chances of using Data Rescue to read the 1's and 0's of "lost" files is greatly diminished.
If you do have another Mac laying around, then you could use Firewire Target Disk Mode to grab files and bypass the whole external drive completely.