nForce 430 vs. intel ICH5 USB hub perfomance

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Hi all.

Few days ago I was trying an USB<->ATA converter.
Using Fedora 9, with an (old) IBM 40GB HD, I got around 30MiB/sec transfer rate with and intel based PC, which has the ICH5 SB.
The very same bridge, with the same HD, still with F9, on an NVIDIA 6150 based motherboard, with nForce 430 SB, results in around 16MiB/sec transfer rate.

Is this normal?
Why the 430 seems to be almost half speed compared to the ICH5?
Is there any possible tuning or testing in order to check what's going on?

Thanks a lot in advance,

piergiorgio