In Mac OS X, why can't I unmount my drive to initialize it?

In Mac OS X, when you attempt to initialize a disk with Disk Utility or a third-party drive formatting utility, you may get an error message about the program being unable to unmount the disk.

This error means that the hard drive can't be unmounted because a file is in use. A drive must be unmounted or taken offline before it can be initialized; if the drive is in use, it cannot be unmounted, and thus cannot be initialized.

You should always boot from a drive other than the one you're trying to initialize. To boot from a CD, DVD, or a different hard drive, see On a Mac, how do I change my startup volume or boot from an alternative volume, disc, or device?

If you are still having trouble unmounting the disk, try the following:

• Restart the computer.
  
  
• Try to unmount the disk manually by clicking the eject button in the Finder. This button should appear to the right of the disk's listing and will look like an up-facing arrow with a line underneath it. If you have the Finder set to display disks on the Desktop, you can also drag the disk to the Trash from there.
  
  
• Make sure there are no applications currently running from the disk you want to initialize. Also make sure no Finder or Terminal windows are accessing folders on the disk.

Comments/Questions/Corrections