cmd-drag file

I want to move a file from internal hdd to ext hdd. sometimes cmd-drag file will move the file. sometimes cmd-drag file only copies the file to the same target drive, which means I then have to delete the original file after copying and then empty trash which adds 2 more steps.

does anyone knows why sometimes cmd-drag move while other times it only copies? same source drive (internal hdd), same destination (ext hdd).