I have several of these for my PCs KINGSTON FLASH MEMORY and have bought them for mac friends. Its important to understand that seagate make a mac specific product KINGSTON FLASH MEMORY that is different in two ways. First its formatted for mac and second it supports firewire. Given these 2 its bigger and more expensive.
If you go the cheap route and buy the PC KINGSTON FLASH MEMORY version you will have to format the disk which erases the pc backup software (for mac they assume time machine). Details on how to do this are on seagate support site at