Copy Problems from WD #A to WD #B

I am using Windows Photo Gallery and Vista with 87GB free space on the main C:\ HD. I have 469GB of scanned photos on WD My Passport Essential SE - 1TB, which I will name as Unit #A. When copying the 469GB and pasting these photos to another WD My Passport Essential SE - 1TB with 900 GB of free space (my Unit #B) —In the beginning, I could copy about 90GB successfully from A to B but then the amount decreased on each copy operation to a lesser amount to where now I cannot copy any photos from A to B.

The copy process starts out with a Dialog Box saying, “Preparing to Copy” and begins showing the number of files it is preparing to copy. Then about 20 seconds later – The Dialog Box disappears without showing me the approximate hours and minutes it will take to copy these numerous picture JPG and TIF format files to my Unit #B.

I do not get any Error or Warning Messages!

Is there some sort of a “Clipboard” involved in the Copy process that might be “Full”?

Also, I originally copied a large portion of my 469GB worth of pictures files to WD #A when that External HD was assigned a Drive letter of “J” automatically by Windows Vista. Sometime later Vista changed the HD letter indicator from “J” to my current “H” drive.

When I attempt to copy a large batch of picture files from WD #A to #B – some of these pictures might have originally been copied in the WD “J” Drive or the current WD “H” Drive. Could this be the problem?  

the problem happens when using two usb drives at the same time

it does not have to do much drives itself… it will rely on the pc performance…

to avoid the drive letter issue… rename the drives… windows will assign the next available drive letter regardless of the drive that has been connected…

since they are similar it might assign the same… that does not mean the same data is been transferred to one or the other… unless you are confuse while transferring … renaming will help with that as well