Sometimes when trying to do stuff with lots of files, I see the subject error. I’ve found that you can sometimes have better results if you are in the directory so that there is no path in your argument, i.e.
# mv /media/old_raw/back/* /media/raw/back/
bash: /bin/mv: Argument list too long
# cd /media/old_raw/back/
# mv * /media/raw/back/
Success!