MAC Operating System Question:

First, make sure that your external hard drive is USB capable. If it uses firewire, then that might be the issue. If it is USB capable, then plug it into the power book. Wait at the most 1 minute for it to recognize the external hard drive. If it does not appear on the screen, you may want to try and plug it in on a second computer. If it doesn't show up on a second computer, the hard drive may have a problem/bug. If it does show up, then drag the selected files onto the hard drive. If it says you can't, then there probably isn't enough room on the hard drive to store the files. That is about all that can be explained to you from the information given.