inspiron 1720 can't read any hard drives

It seems like my Inspiron 1720 (still under warranty) is falling apart, but I can't determine the root cause. Recently, I had a lot of trouble with USB/power/audio ports, including some possible looseness, so Dell sent a technician to replace my chassis, motherboard, HDD caddy, and most of my ports/cables.

Things worked OK for about a week but now I'm having serious HDD problems. I've been trying to upgrade my HDD, using software to image my drive to an external HDD and then "restore" to the new installed drive. In the past, with this laptop, I have never had problems with this. When I tried this time, however, I got all sorts of errors from multiple pieces of software... things would be going OK and then there'd be some sort of disk-related error.

Eventually, I successfully imaged it, and restored to the new drive. The restoration took FOREVER (maybe 60hrs) even though I was only copying a 320GB HDD. It seemed that although the Inspiron's disk light was on, it wasn't running at full speed.

This is important, because since then, with both my "old" and my "new" drive (both now have images), I have seen identical behavior. The Inspiron 1720 HDD light will light up, but the drive won't be running at full capacity / at all. I'll only hear the clicks every once in a while.

Now, today, it seems the HDD connection has completely given up the ghost. No matter how I seat either drive, I get the error "A disk read error occurred" when I try to boot. When I boot to the Dell Diagnostics CD, the drives fail the confidence test with the message "timeout waiting for drive not busy."

I know that these drives aren't both bad, and most likely neither is bad. As recently as earlier in the week, I ran Dell Diagnostics on at least one of these drives and got passes for everything, and within the past 24hrs I ran dskchk on the older one and had 0 bad sectors.

Given that Dell has already replaced the motherboard, chassis, HDD caddy, and a variety of ports/cables, what else could it possibly be? Is there a separate hardware HDD controller? Dell didn't replace the interpolator but that seems unlikely to be the cause since it's just a piece of metal, right? Thanks in advance for your suggestions.