Linux Error Message: inter-device move failed, unable to remove target: Is a directory

 Trying to move some files from a network share to a USB drive on the Linux terminal and came up with the following unexpected error message:

inter-device move failed, unable to remove target: Is a directory 

In my case it turned out to be that the destination folder already existed. By changing the destination folder to a folder that didn't exist and the move worked straight away. It's weird because I would've assumed that the default behaviour would've been to merge the contents of the two folders and delete the source folder. I'll do some more research on this one when I get the chance.