Error while copying files to Hard disk

I am getting this error while copying my files to Western Digital 2TB external hard drive via USB.

I am getting error sometimes when copy is done 30% or 60% or 90% and I am getting this frequently.

ERROR : error 0x8007045D : the request could not be performed because of an I/O device error

I am also checking here but I wanted to get more insight in what actually the problem is ? Please help, I am using Windows 8 system, I see this error again and again for different different files. How to fix this?