Every ST3120026A was 3-head and 80GB/disk. Every ST3200822A was 4-head and 100GB/disk…but out of nowhere, Seagate chose to never list the head counts for the 7200.8 in any of their documentation. StorageReview said that the 250GB and 400GB models used 133GB/disk, whereas the 200GB and 300GB models used 100GB/disk, giving even head counts for all models. (Using 133GB/disk for the entire series would give 3 heads for the 200GB model and 5 heads for the 300GB model; saving the 133GB disks for the 400GB model alone would give 5 heads for the 250GB model. Some compromise was understandable at the time, as 7200.8 was the first drive with such a high density.)
7200.9 had more to it than meets the eye. If you dig deep enough into the (discontinued) Seagate Desk Reference, you'll find…wait for it…
…the ST3500632AS!
To be complete, there's a whole “set” of unreleased 7200.9s (all with the same areal density specs):
ST380210A(S)
ST380810A(S)
ST3120210A(S)
ST3120810A(S)
ST3160210A(S)
ST3160810A(S)
ST3500832A(S)
ST3500632A(S)
Although they didn't explicitly list the head counts even for those, it does suggest that they tried to squeeze 500GB onto 3 disks but failed.
So they went to 4 disks for the 500GB model, but that's not all. They pulled a DiamondMax Plus 9* and used various configurations in models from 200GB to 400GB.
(Originally there were links to pictures of the drives, but they don't work anymore, so have been removed now. But suffice to say that some 400GB 7200.9s had four disks [were they 100GB each? Or maybe 125GB each but short-stroked?], which was actually a step backward from the previous model.)
Only the 80-160GB models retained their extra-high density (though they're not quite the same as the unreleased versions). The (original) 40GB model was a different design with lower density than any of the larger models. (In the slimline redesign of the single-platter models, the 40GB model was a short-stroked 80GB, just as the 120GB models in both versions were short-stroked from 160GB.)
*Maxtor's line which was originally intended to be 80GB/disk throughout, but suffered from yield problems. So they started making drives with the same model number, but different areal densities. It also wasn't renowned for reliability, but that was another story…
(In case you're curious: Another set of unreleased drives listed in the Desk Reference was a trio of 5.1GB/disk Medalists - ST320440A, ST315330A, and ST310221A.)
EDIT: Revised 2014/12/15 (removed the broken photo links, minor wording alterations)
