Windows 7: copying files to external HDD

hey guys i noticed something really strange. i tried copying three iso files from my laptop to my external HDD. the problem is that after one iso is copied the copying process stops and resumes again after a pause and after that it pauses again and the third file starts. why it pauses to copy? i tries usb 3.0 and 2.0.
in usb 3.0 the spped is around 94-95K Mbps. but once it starts to copy the second iso it drops to around 30K Mbps.
its same with both the external HDDs.
whats wrong?

thank you

Copying and pasting files via Windows explorer is always going to be a bit inefficient. There's a lot for the machine to do when explorer is in the way. There will be pauses on big files, especialy .iso files. (at least Windows 7 is better than Vista's copy/paste, which was just horribly slow)
Have you tried to do the copy job using RoboCopy instead of Copy/Paste in Windows explorer? RoboCopy is much more efficient. However, big .iso files & other big files will still pause during the run.

i tried using total commander but the pause is still there. thanks for answering.