Below is a GNUPlot graph of ZCAV output from a 250G SATA disk. The disk has something wrong with it (other disks of the same brand in machines of the same configuration give more regular graphs). The expected graph will start high on the Y scale (MB/s) and steadily drop as the reads go to shorter tracks near the spindle. The ZCAV results page has some examples of the expected results.
If you have any ideas as to what might cause this performance result then please make a comment on this blog post.
Update: This problem is solved, I’ve written a new post explaining the answer. It’s a pity that an NDA delayed my publication of this information.