Hardware :: Disks Speed Varies When They Have Same Amount Of Data?

My machine has 4 SATA 2 West Digital 1TB disks. I made 2 partitions on each of them, 500GB for each partition. When I started using them I check their I/O using iozone. The first partition has 100MB/s for read, 70MB/s for write. And the second partition has 80MB/s for read, 55MB/s for write. All 4 disks has the same result.

As I use on, the I/O speed on each partition decrease, to different extend. For example, for the 4 first partitions, the write speed varies from 69MB/s to 56MB/s. And I have same amount of data on each of them, all used 11%.

My guess for this is the disk block allocation policy. This is caused because some disk starts writing from inner location while others writes on the outer edge, even though amount of data on each disk is the same.