Andrew,
Yes, the ICH9M-E uses the Intel Matrix Storage Manager software. I have
contacted Intel, and they do not think the controller could cause this
behaviour.
In any case, as I said, I can reproduce the same behaviour on an external
USB drive, which presumably has nothing to do with the ICH9M-E whatsoever.

I have no reason to suspect that the drives are "failing". I am seeking an
explanation of the behaviour I am observing. I am especially interested to
know why, when I disable processor speed throttling in the power management
profiles "Minimal Power Management", and "Maximum Performance (Dell)", the
slow io response times occur again. It appears to me that disabling
throttling also changes some other setting, which I have no obvious control
over.

I will nevertheless investigate those two downloads - thankyou.

Greg.