First get Acronis True Image or Disk Director for XP/Win7. Fast and dead easy to use. I don´t know if there´s a counterpart on Mac, on board OSX tools weren´t able to change anything on my Seagate FreeAgent GoFlex 3 Tb.

1. Install Acronis & Start App
2. Plugin your new ext HD
3. Erase your new ext 3 Tb HD
4. Format with FAT32 and 64k cluster size
5. Go to the kitchen and get yourself a yoghurt
6. Unplug from PC and plug into PS3
7. Buy the games you love.

cheers
ElD