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If I take a 500GB hard drive and a 1TB hard disk otherwise identical, why is the 1TB drive not twice as fast as the 500GB drive?

If they are both spinning at the at the same rate, and the the 1TB drive has twice the data density (as it must), it should have a transfer rate of twice as much (although seek times will be comparable) or am I missing something.

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Some useful info has been posted here (the antithesis of this question): superuser.com/questions/21486/are-smaller-hard-drives-faster – sblair Nov 7 '09 at 2:34
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Hard drives don't necessarily differ in the data density but also in the number of platters, for example. It's not uncommon for one capacity jump to just have double the number of platters instead of double the data density.

Furthermore, there are several factors affecting the data transfer rate you get from a hard disk. The most prominent one is probably the controller in the drive itself. If that one is limited ot maybe 80 MiB/s then more data density gains you nothing.

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If there are twice as many platters, why cannot they be striped - they are all rotating at the exact same speed - would it not be trivial? eSATA2 supports transfer rates of upto 3GBits/s which would roughly translate to 300MBytes/s, if the 500GB drive runs at this 80MB/s, then the 1TB drive should be able to achieve up to 160MB/s. – Alister Oct 26 '09 at 23:30
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The drives could only max-out the 3GB/s SATA limit during a perfect (defragmented) sequential read, of a very large file. This only occurs very rarely, so the realistic speed is worse. And every time the drive needs to do any random read/write, or move position, there is a relatively huge delay for seeking. – sblair Oct 26 '09 at 23:43
@Alister: I believe most drives will stripe their data across platters to maximize performance. Trying to do so in the most optimal manner for small or fragmented files doesn't hold up very well though. – Jimmy Oct 27 '09 at 6:40
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