If you are having problems with a disk (e.g: your computer is running slowly), you can check if your disk is OK with Disk Utility.
Disk Utility is found in the Utilities folder in the Applications folder on your disk.
You can select a drive from the list at the left after opening Disk Utility.
First Aid
The first thing we can do is go to the First Aid tab. This allows you to verify or repair a disk. If it is a Mac OS X startup disk, you can also verify or repair the permissions.
If you cannot use First Aid but rather see this message, then it's a hardware problem with your disk. The S.M.A.R.T status will also read "Failing" in red, and the failing disk in the sidebar will appear with its name in red. You should replace your hard drive; make sure all your data is backed up!
Otherwise, select the disk with problems in the sidebar. In the First Aid tab, this interface will appear:
My first recommendation is to click "Verify Disk". This will check the disk for problems.
(i) If the disk needs repairs:
(i) If this is your startup disk that has problems, use Disk Utility from another volume. If you don't have another bootable volume, insert your Mac OS X installation disc, select your language, and then choose Utilities > Disk Utility and repair your disk from there.
(i) Otherwise, continue following this guide.
Click Repair Disk. This will do a Verify action and Disk Utility will try to repair any errors.
(i) If you get the above message, and the repair fails:
(i) Back up as much of the data on the disk as possible.
(i) Erase the disk by following the below instructions.
Erasing
"Erasing" a disk is the term to deleting all the data on a disk.
To erase a disk, click on the Erase Tab, which open this interface:
You can choose a format from the pop-up menu. You can also enter a new name for the drive.
(!) Erasing a disk will delete all the data on the disk. Make sure you back up your stuff before you erase any disk.
First, lets go to Erasing Free Space.
When you drag a file to the Trash, they are moved to a location where files can be previewed, but not opened directly. When you empty the Trash, it just deletes the information used to make them online on a computer. Some file recovery programs can restore that data before you erase the free space.
Zero Out Data: The computer language is made of 1's and 0's, right? Well, zeroing out data causes even the existence of the offline files to be deleted permanently. They cannot be restored except from backups. "Zero Out" comes from the term to write zeros (replacing the 1's with 0's) all over the data.
You can also choose 7-zero outs or 35-zero outs. This also is with clicking the Security Options button.
"Don't Erase Disk" in the Security option is "general Trash deletion". This is where files are moved to the trash and then emptied.
If the Erase button fails and First Aid reports problems that DU can't fix, you know the drive itself has probably failed.
