Windows 7: Can't defrag external HDD, it sometimes runs very slow.

Hi, I've got a samsung 3.0 external USB drive connected to my computer trought PCIE USB 3.0 adapter and it regulary works perfectly but sometimes, when it comes to copying big files ot cutting some folders it causes the system to freeze till I restert the explorer.exe. Also there's a problem if I want to analyze the disk using disk defragmenter it freezez at 0% and I can't cancell it because it stucks at "cancelling". Any advices?

Go to start/all programs/accessories
Right click COMMAND PROMPT
Click RUN AS ADMINISTRATOR

Type this

chkdsk x: /r

Replace the x with your drive letter

Has the drive been dropped or hit while running? Is it a sata drive connected through a usb 3.0 port?
Another thing you can try is updating your drivers .. for the usb controller
If you copy large files often, format the drive with bigger cluster size things

alternately go to computer right click the actual drive and select properties/ tools tab and check the disk there in error checking.