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.
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
Actually, the capacity is proportional to the area, whereas the transfer rate is proportional to the radius. Assuming that the number of tracks increases in proportional with the amount of data per track, the transfer rate should increase with the square root of the increase in capacity.
So increasing from 500GB to 1000GB you double the density per area of the drive, but the density per track is only increased by a factor of 1.414 (the square root of 2), so the transfer rate is only increased by a factor of 1.414.
I suspect the real factor may be a bit higher because it may be easier to increase the density per track than the number of tracks.
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.
