Why does the speed decrease while copying from an external harddrive?

I have an external hard drive that I needed to copy about 30 GB from. 8 DVD image files. The copy started out at 160 MB/s, quickly went down to around 80 MB/s and then slowly during the process it has now gone down to 11 MB/s (2.7 GB left)... What is going on here? Why does this happen?

Using Windows 7, copying from an NTFS formatted USB drive, SAMSUNG HM160HI USB Device. As far as I know it is connected to a USB 2.0 (otherwise I think Windows would have complained?) and it is platters, not solid state.

I'm getting the transfer speeds from the regular Win7 copying dialog (after clicking the Show Details button)

Solution 1:

OS and drive buffers have filled up/been exhausted, and now you're transfering data as fast as the hard drive can supply/consume it.