Strange Data Transfer Behavior

I have a strange case here .
I can transfer every thing from Drive to drive with no problems , except for some Media file . I have 5 of them . They're video files ( BBC programs about Science ) . So i started to notice when i transfer more than 100GB of Video from one HDD to another one . Everything was fine , until these files are being copied , speed dropped down less than 8MB/s . I tested again and again with other HDD . Its still the same symtom . So i try to copy other files first , and everything is OK !!!! But when i choose these 5 problematic files , it happens again . Although these files are still normal . I can watch them , navigate them normally in Video Players .
What is happening ? How can i fix it ?

Thanks :)

mkv . I did another test with these file . I have 2 copies of them , on 2 HDD . first i copied them from A to B . A is the one i downloaded them , B is the one i make a backup . The Low transfer rate occur when i copy A -> B . Then 1 month later . I copy B -> C , the new one i've just bought this evening , same thing happens , then i posted this thread . Minutes ago , i tried copy again , but this time from A - C . It was fast ! Then , could it be that B has some bad sector that i happen to copy data over them ? Is there any software to check it ? and fix it if possible ?