Is the HDD causing computer slowdown?

Yes, the hard drive can slow things down very badly.

I had an ten-year old laptop running Windows XP, the processor was an AMD Turion 64 Mobile ML-28 which I think would be slower than your 1.6 GHz Pentium dual core.

I replaced the hard drive for more space, I wasn’t really thinking about speed. I cloned the XP installation onto the new drive, so there were no changes other than the HD.

The machine ran MUCH faster. For web browsing, Excel, it was perfectly suitable for every day us. I would still be using it if the power connector hadn’t melted.

Generally the higher the density of the HDD the faster they can read and write. If you think about it both the 40GB and the 1TB drive are the same size (3.5 inches) but in order to store more info on the disc the 1TB drive writes much smaller (it probably also has several discs so is reading from several discs at the same time). Newer drives also have larger caches of RAM so data can be transferred to and from the drive faster than the drive is actually storing it. The other factor that can effect drive speed is the connection between the drive and the motherboard. The fastest SATA connection at the moment has a speed of 600MB/second so can transfer data fasted than the hard drive can read it. The original SATA spec (from 2003) could only transfer data at 150MB/sec which is still faster than most HDD’s can read or write for a sustained period. Before SATA there was IDE (which had been knocking about since the 80’s and you’d still see well in to the 2000’s) which was even slower.

So probably the old drive is operating much slower than the new one but whether that alone is enough to make it seem so slow as to be unusable is a matter of perspective. My PC at home uses SSD’s and boots off a drive connected via a PCIe slot. This makes it ridiculously responsive. Using PC’s at work, particularly older ones, that are still using XP can be particularly painful. For example booting takes an age (3–4 minutes compared to 5–6 seconds for my home PC).

I’m trying to understand the process here first.

You have a relatively new computer that now has a 1 TB HDD salvaged from an older machine (machine #2), and the new drive is encased and external to the new machine. That’s not the problem.

You also have a 40 GB HDD from a third machine, which you have replaced instead of the 1 TB HDD that was taken from machine #2. Machine #2 with the 40 GB drive from machine #3 is the problem. Is that right so far?

If so, then when you replaced the 40 GB drive where machine #2 had expected to find the former 1 TB drive, did you re-run the setup routine to identify the new drive to the machine? (Is that even necessary to run manually any more? It’s been a long time since I’ve played under the hood with desktop machines and swapped out drives.) I don’t know what the computer processes and logic are, but if machine #2 is still trying to address a particular 1 TB drive and finding the 40 GB drive, it can be doing a lot of searching for sectors that it (obviously) can’t find to read or write to.

It’s worth taking a look at during the machine’s boot cycle (hit whatever keystroke combination the monitor says to run “setup” at a cold boot) to see if the plug-and-play worked to properly identify the “new” drive to the machine.

A twelve year old drive is dramatically slow compared to newer ones. Yes, that’s probably the issue.

As others have said, your HDD is causing the issue. If you have some money to spare, I’d suggest getting a SSD that’s big enough to hold windows and some essential programs. Everything else you can just on your HDD. You’ll be amazed at how much faster this will make your computer.
(mine takes longer to POST than it actually does to boot.)

As @ARE_you_kidding_me (“dramatically slower”) and everyone else notes, a 40GB drive will be generations of technical progress behind a contemporary drive. Check its specs for seek times, data throughput, and caching and compare those specs to anything currently available.

I shudder to think how abysmally slow that 40gb drive is compared to anything made in the last 10 years. They stopped making hard drives that small around the time data transfer rates got fast enough that you didn’t have time to make a sandwich while the computer booted. And I have applications that wouldn’t even fit on that drive.

So yes, using last-century technology is slowing your computer down. At best, it has half the transfer speed of a modern budget hard drive over 10 times the size; probably closer to one-quarter as DMA/66 was optional then (DMA/33 was more common) while even slow SATA drives are too fast for DMA/100, and some faster than DMA/133.