"If you bought an external hard drive to use with your Mac, do you just plug it in the way it came, or should you reformat it to work with the Mac?" John Carter gets our attention. " If you bought the drive to use with Time Machine, then Time Machine will by default reformat the drive. But, does it need to be reformatted if you’re not going to use it with Time Machine? And what about that really big 2TB hard drive? Should you leave it as one big partition (the default), or should you repartition (and reformat) the drive into smaller pieces?"It would be an injustice to restate what Ken Stone says about partitioning hard drives for a Mac, so I’ll just include this quote from his website to entice you to read the rest of the story in his website: '...whether you intend to partition your hard drive or not, you really should reformat your new hard drive for the Mac. If your new hard drive is FireWire and you intend to use it with both Macs and PCs, then leave it as is and do not reformat.' John continues, "Apple walks you through the process of how you would go about repartitioning, and why you would want to, in this fascinating article." Looking it over I sent a few questions to John and he further reports, "The article discusses Leopard, but not Snow Leopard. I’m sure it will serve for Snow Leopard. As to more recent external drives, if the interface is Firewire 800 or 400 or USB 2.0, it will work. If the interface is USB 3.0, the Mac isn’t compatible with that format. Some USB 3.0 drives claim to be backward compatible with USB 2.0 and work on a Mac. I have to see it to believe it."