Yup. Seagate was #1 for years, but when Maxtor merged with Quantum, Quakstor went past them by a fair margin. So this is quite a significant change: Seagate, it seems, are back to #1.
WD are well behind Seagate, Quaxtor, and Hitachi, probably in front of Samsung but I'm not sure of that last. Samsung, remember, are strong worldwide, especially in Asia, whereas WD are very strong in the USA and weaker in the rest of the world.
But WD are much smaller than the three giants: Seagate, Maxtor & Hitachi. Hitachi, BTW, are probably quite a bit smaller than the old IBM was in its glory days.