Works with Mac or PC. Formatting the entire 1TB to FAT32 with a Mac allows to use both Mac and PC and transfer between them [although you can transfer via a home network router using the Mac]. Files size is limited to around 4GB with FAT32 though. However, you can always use the Mac to paritionfor OS Extended and another partition NTFS. Leopard and Snow Leopard can read NTFS, but I'm not sure if it can write to it yet.