SATA drive operating very slow.

Installed opensuse 11 ( tried both 64 bit and 32 bit) on Asus M2A-VM MB and AMD Athlon 64 X2 5000+ CPU. The drive is seagate 500G 32M cache, SATA 3 g/s.

It uses ahci driver ( I also tried switching the controller to IDE). It disables the 64bit DMA and something called MPM or something. With hdparm, I get about 45 MB/sec. On the same MB, an IDE drive with 8MB cache gives 55MB/sec !

Just to try a newer kernel, I booted with mandriva one 2009 RC1 and hdparm on the sata drive showed 105-110 MB/sec !

Why is opensuse not able to get this performance out of the HD ?