You should run workshops on "How to describe your issue and make it sound like fun to troubleshoot". Have rep
Did you have a chance to test safe mode, as discussed?
What exactly are these "external drives"? USB sticks? HDDs connected via USB? HDDs connected via 1394 (firewire) or eSATA? Networked drives? If you haven't already done so, it would be very useful to verify whether the symptom is confined to one or more of those device classes, or whether it includes all of them - even a lowly $4.99 256MB USB stick.
Which process do you find yourself needing to end-task because it's consuming processor time? For example, if you're copying via Robocopy, does Robocopy become the offending process which you're aiming to kill if you want your responsiveness back?
Does this machine have a functioning serial (RS232) port, or 1394? If so, do you have another box close by which you could link to this one via a cable? (In a "last resort" situation, that type of setup can be used for some in-depth troubleshooting.)
I'm unclear on whether you've observed this issue first-thing-after-a-reinstall of the OS? In other words, was the symptom under Win7 always there, even before you apply any updates or install AV utilities, apps... ?
The HAL is literally one of the lowest-level and most important parts of the OS, so it's impractical at this juncture to speculate as to whether the HAL is somehow the culprit. It's definitely involved (in everything), so it could potentially be in need of an update, but running an incompatible HAL can be anything from frustrating to unworkable. For now, I'd suggest you don't experiment with HALs.
I'll try to make some more practical suggestions once I have a better grasp of the scope of the problem.