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The 640GB Western Digital Caviar Black is one of the few internal hard disks we’ve seen to use the new SATA III interface. Once rare, SATA III is now included on an increasing number of motherboards. If you have one of these motherboards then you may be tempted to buy a SATA III hard disk, which, on paper, should be twice as fast as a SATA II disk. Our test results of the Caviar Black should give you reason to think twice before doing so, though.
We tested the Caviar Black with a Gigabyte SATA III-equipped AM3 motherboard and were impressed by its large file transfer speeds. Large files were read at a chart topping 122MB/s, which even exceeds the speeds of some SSDs. Large files were written at 113.6MB/s which is also very fast, but some SATA II disks performed almost identically. Small files were read at 98MB/s. Again, while this is fast, other SATA II disks can easily match this. Small files were written at 64.6MB/s, which is a respectably quick score but lags behind the fastest SATA II disks. 
Although the Caviar Black can be used with a motherboard that doesn’t have SATA III at slower SATA II speeds, we don’t see the point of this even if you plan on later upgrading to a SATA III capable motherboard. The Caviar Black isn’t consistently faster than the best SATA II disks. It’s expensive, too, at 8p per gigabyte. A larger SATA II disk is better value for most people.
