New hard drive-intermittant copy operation

I just bought a 500 GB Seagate Barracuda SATA internal HD. I installed it in my new Acer desktop computer running Vista Home Premium, 2 GB RAM, duo core processor, on SATA2.

When I was copying about about 5 GB of files to it from the main internal hard drive, I noticed that at first it said time remaining was something like 8 minutes. But about halfway through the copy, the progress bar stopped moving, and the time remaining climbed up steadily til it was about 30 min. No progress for about 3 minutes. Then the progress bar started moving again, finished the file copy in about a total of 12 minutes.

I initiated another copy, this time from the new hard drive to the new hard drive (I wanted to eliminate the other hard drive from the picture), opened Vista's Resource Monitor, and sure enough, the hard drive would occasionally stop transferring data for gaps of about 2-4 minutes, every few minutes.

When the gap in data transfer is happening, my CPU utilization is very low, about 1 to 3%, so I do not think any other activity is going on that would cause this.

The other drive, on SATA1, does not exhibit this behavior when I do a copy in the same manner.

I returned the drive, got an exact replacement, it exhibits the same behavior.

But when I put the drive in an external enclosure and connect it via USB to the same computer, it does not exhibit this behaviour, it runs fine.

Any ideas?

Thanks