Modern drives have variable sectors per track and variable tracks per inch. These numbers vary between heads on the same drive.
See the following article:
http://hddscan.com/doc/HDD_Tracks_and_Zones.html
You can see the differences between each head in a short-stroked HD Tune read benchmark graph:
http://malthus.zapto.org/viewtopic.php?t=650&p=2591#p2591
The same thread shows the actual geometry of a Seagate drive. WD’s drives would be similar.
