Here are direct links to your images:
http://i.imgur.com/kLo84.png
http://i.imgur.com/j3P7j.png
http://i.imgur.com/pu9Z4.png
AISI, the “slow” WD drive and the Samsung drive both show the typical 2:1 ratio between the transfer rates at the outermost and innermost zones. This is to be expected since the diameter (and circumference) of the outer and inner tracks is in the same ratio. That is, the innermost zone has half as many bits per track as the outermost zone, and therefore half as many bytes pass under the head during each revolution. The WD graph also shows 16 or so steps corresponding to each zone.
If you now compare the “fast” WD graph against the slow one, it appears that there may be only two or three zones. In fact the transfer rate at the 100% mark on the fast graph corresponds to the 35% mark on the slow graph. This suggests that the fast drive has been shortstroked, and that its full-stroke capacity is more like 914GB (= 320GB/0.35). Therefore I suspect that the fast 320GB drive is actually a 1TB drive that has had its capacity truncated. Shortstroking would also explain why its access time is significantly better.
Another clue as to what may be going on is the DCM (Drive Configuration Matrix). The following thread would suggest that there is a difference in the head stacks of the two WD drives:
http://forum.hddguru.com/western-digital-what-dcm-t6488.html
I would think that the 1TB drive would have 3 platters and 6 heads, whereas the 320GB drive would have 1 platter and 2 heads.
One final observation I’d like to make is that the maximum sustained transfer rate (105 MB/s) is a lot less than what is touted in the datasheet (126 MB/s).
http://products.wdc.com/Library/SpecSheet/ENG/2879-701277.pdf
In fact 126 MB/s is what you would expect from a 7200 RPM drive with 500GB platters. Instead your drives appear to have 350GB platters (333GB ???).
((105 / 126)^2) x 500 = 347
