Keep in mind that's megabits per second (Mbps) and neither comes even remotely close to the 3.0 gigabytes per second (GBps) rating of the serial ATA hard drive inside your box.
A third option, FireWire 800, is rated at a significantly better 800 Mbps, but PC owners will be hard pressed to find a complementary port as standard equipment on their computers. It is, however, Mac friendly and, as Apple increases its penetration into the marketplace, you'll probably see more of it on external drives.
A fourth option also has begun to appear: external SATA (or just eSATA). This is the same 3.0-GBps interface that's inside your PC brought outside the box. Unfortunately, few, if any, external hard drives have an eSATA interface included in their boxes. At best, they present you with an eSATA cable -- which has different connectors on the ends than the SATA cable used inside your PC. eSATA leaves you with three possibilities if you want to use it as your interface:
Your computer already has an eSATA port;
You buy an eSATA interface card;
You buy an eSATA adapter cable that runs from an internal SATA port to an eSATA port on a bracket that mounts to the back panel of your PC.
Naturally, if you have an existing eSATA port, you're on easy street. It's a plug-and-go solution (although you'll usually need to power down your PC first so it will recognize the added drive).
Whether to buy an eSATA card depends on your computer's configuration. In these days of scalable link interfaces and crossfire graphics (as well as graphics cards gorged with heatsinks, fans, and/or dual graphics processing units), you may not have a spare PCI slot available. It's always best to check first.
The adapter cable route is a bit more esoteric. Basically what you're doing is running an "extension cable" from a SATA port on the motherboard to an eSATA port on the outside. Typically they're available from Newegg or Addonics for $4 to $9 and often come with the back panel bracket -- but sometimes don't. The downside is that you will need to crack open your PC and make the connection to one of your motherboard's SATA ports -- which are often directly beneath the most dense entanglement of other cables and wires.
If you happen to see a USB to eSATA adapter and think it's an easy out, forget about it -- unless you're looking for USB transfer speeds despite having an eSATA option on one end of the food chain. (It's that "lowest common denominator" rule at work.)
Also, bear in mind that "transfer rates" are theoretical and highly optimistic. The throughput speed (that is, the actual rate at which data travels between your PC and external hard drive) depends on differing factors -- some of which are controlled by your computer -- and is often lower than the transfer rate.
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