From what I've read, the earlier hybrid drives required the OS to be involved, and apparently only Windows supported it.
Update: with more real-world testing under our belts, Seagate's Momentus XT is receiving our rare Best of Tom's award for excellent performance
Hybrid hard drives (H-HDDs) arrived in 2007, and they disappeared soon after because there was hardly any real benefit. Today, Seagate unveils a new type of hybrid that is similar to the initial H-HDD concept. However, the new Momentus XT does not rely on operating system support, and hence should be more versatile.