- Connect the hard drive or USB key to the Mac
- Launch Disk Utility, located in Applications > Utilities
- Locate the drive name from the left hand side of Disk Utility and click on it
- Click on the “Erase” tab across the top
- Next to “Format:” click on the menu and select “MS - DOS (FAT)" [Fat is read by all systems, if you only want it to be readable by a mac you can choose “Mac OS Extended (Journaled)”
- Name the drive (the name can be changed at any point)
- Click “Erase” and confirm again on the next pop-up window
2. Back up to an external hard drive:
Click "Instructions for backing up to an external hard disk via Disk Utility"
http://support.apple.com/kb/ht1553
3. Reinstall OSX
http://support.apple.com/kb/PH11273