Currently Being ModeratedJun 23, 2013 2:34 PM (in response to Alieufromwa)
Check the formatting of the external hard drive. To do that, right click on the drive icon and select Get Info. (Or click once on the drive and type command +i ) The pane includes information about the drive format.
The drive should be formatted as Mac OS Extended. If it is not, run Disk Utility (App;ications / Utilities) and format the drive.