Since i can't really explain it, i'll try illustrate a mock up scenario where the problem happens:
1. Your fiend is over at your place and you're impressing him with your amazing video library.
2. He decides that he wants a selection of what you think are your best videos
3. He gives you his external hard drive to put the videos on.
4. You make a small selection of files, and start copying them to your friends external hard drive.
5.You're getting fast speeds and everything's fine, until you find more videos that your friend will love to have. So you start copying these videos too.
6.Instead of adding these files at the end of the initial file transfer queue, a new file transfer instance is created. Speeds seemed to have evenly divided between the two instances. However you want to rename a few files on the external disk only to find that the performance of the external disk has suffered a major blow, but still usable,
7. You then find more video files that you want to copy to the external drive. You start copying these files too. A third instance of the file transfer dialogue appears and speeds severely suffer.
-The Disk Queue Length in the resource monitor is maxing out.
-You can really hear both your internal hard disk and the external hard disk working really hard.
-You then realise that your internal hard disk is been asked to read data from 3 phyisically different locations on the disk platters at the same time, likewise for writing to the external hard disk.
-The sum total of the 3 file transfer speeds is much lower than the speed of the single transfer in the beginning. This also means that it will take a longer time than if you waited for the first lot of files to finish and started the next lot
-Using the hard disks like this is more than likely to be shortening their lifespan and reducing their ability to continually deliver high performance.
I hope this illustrates what i'm trying to get at here.
Simply add the ability to queue transfers.
Fixed in Windows 8.