Unable to Copy WD#1 to WD#2

I am using 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 operartion to a lesser amount to where now I cannot copy any photos from A to B.  

Uhm, are you in the correct forum?   This forum is for the WDTV Live Streaming Media Player.

Tony > What would be a more appropriate Forum to transfer my inquiry to? Thank U from Bill L

Possibly the External HD’s forum: