Basic hard drives are cheap enough to backup your whole 500GB Macintosh hard drive for a under $100. Spend a little more, and you'll get niceties like 1.5-3TB of drive space (to keep more vintage backups in Time Machine), FireWire 800 (for faster transfers), Thunderbolt (fastest transfers), or even extra features like charging ports for your iPad. Which brings us to the best justification for backing up your Mac: Time Machine is dead simple. Just plug in a new hard drive, and as long as the drive is larger than your internal HDD, Mac OS X will ask you to use the drive as a Time Machine backup drive. Say yes, and everything is handled automatically in the background. If the drive is a portable with USB 2.0 or FireWire 800, it's even simpler: you only have to plug in one cord.
Other features like external capacity gauges are better suited to drives that you're copying to manually, like the drive you use to transfer large files from one Mac to another or to a PC. Other features that may not be relevant for you average Mac user are USB 3.0 (There are no USB 3.0 ports on Macs as of yet), eSATA (ditto), and packed in backup software (Time Machine is fine for basic backups). That said, if you need to keep a certain folder backed up and brought offsite, file-based backup software can be helpful.
How about FireWire 800? If you have a MacBook Pro, Mac Pro, iMac, or Mac mini, the answer is yes, you should choose a drive with FireWire 800. On the MacBook Air, some MacBook Pros, or older MacBooks, the answer is "not really", since they lack FireWire 800. FireWire 800 should be your interface of choice if you're planning on creating an emergency boot drive, since Mac OS X has greater affinity for FireWire 800 for booting older Macs. Recent Macs will boot fine off of a USB drive.
Thunderbolt is another consideration, and is a good choice if you use your Mac to earn a living. Thunderbolt is simply the fastest direct attached interface right now (dual channel 10Gbps vs. 480Mbps for USB 2.0), and you can even stream multiple HD videos from a Thunderbolt drive while driving a 27-inch monitor from the same Thunderbolt port on a Mac. The factors that are preventing a wider adoption of Thunderbolt are the current high price of drives, the limited selction of drives at the moment, and the single Apple-sourced Thunderbolt cable, which is a bit pricey at $49 each. We recommend Thunderbolt for the graphics arts and engineering business users that absolutely need their work done now. Besides, businesses can write off the extra cost of the drives over time.
So, yes, you can buy just about any USB external hard drive and reformat it to use on your Mac, but whether you have an older Mac or a Mac with a fresh glass and aluminum body, it behooves you to get a drive with the fastest interface possible. For pre-2011 MacBook Air and MacBooks, you want a USB 2.0/3.0 drive, for pre-2011 MacBook Pro and desktops, that's the FireWire 800 model, and for all new Macs (except the Mac Pro) since early 2011, a Thunderbolt drive is the fastest. Thunderbolt will become more reasonably priced in 2012, but there will still be a price premium for speed. In any case, having a Time Machine drive can mean the difference between success and failure if you ever lose a file, so backup your Mac.
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