James, eSATA has a maximum throughput of 3Gb/s. That’s gigabits, though, not gigabytes. So, figure around 300MB/s, less overhead. However, you won’t see many mechanical hard drives actually reach those speeds—the fastest consumer-level hard drives barely get 200MB/s sustained read speeds, and a 250GB/5,400rpm drive doesn’t even approach that. You’re constrained in this case not by the transfer speeds of the eSATA connection, but by the mechanical speed of your hard drive. Especially if you have an older drive, 40MB/s sounds pretty reasonable for a 5,400rpm device. But look on the bright side: It’s still faster than you’ll get using the USB 2.0 interface.
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