>
> An added note is the the system improves dramatically with netBSD 1.6 and both PC's operate as expected. However I do not want to upgrade to 1.6 just yet and would like a solution for 1.5.1, if possible.
>
possibly NetBSD 1.5 doesn't fully support this new IDE controller while
1.6 do so.

please look at dmesg output if this harddrive use DMA at all.

to measure transfers

dd if=/dev/rwd0d of=/dev/null bs=64k count=2048

is useful (on really fast drives try higher count).

please do not mail with almost no line breaks. it could be (and is)
annoying.