The World Wide Geek demonstrates how to format an external hard drive for Mac OS X. External hard drives are usually formatted for windows. To change the format, you will need to use the disk utility option. Access the disk utility by going to finder and clicking on applications. Next, click on the utilities folder. Then, click on the disk utility option to open it. You can also search for disk utility in spotlight. Select your external hard drive in the options to the left of the disk utility window. Click on the erase tab and select the Mac OS Extended option. Enter a name into the appropriate field and then click the erase button. Your external hard drive will now work with Mac OS X.