Copying files to NAS gives Error Code -50 "Error in user parameter list"
The problem for me was that there were several invisible .appledouble folders within the folders I was trying to copy.

These folders are written by OS X for compatibility with Apple Finder Metadata with some NASs but are safe to be deleted (at least in my case). I just removed them all and everything was back to normal.

You can automate the process by typing the below into terminal when your pwd is the external drive. Please only do this if you are comfortable with the terminal - otherwise you could end up deleting important things.

find . -name \.AppleDouble -exec rm -rf {} \;