Seagate is already associated with Apple, since Apple already use their drives, and so it would be really easy for them to draw up the contracts for the new line. Also, Seagate are probably the biggest manufacturer of hard drives (especially with them now owning Maxtor), and so they would more-than-likely be able to offer them to Apple at a far more cost-effective price than any of the manufacturers you've listed.
onlyone-jc.