It’s pretty common to have to move files between two hard drives when you’re importing and exporting projects or backing up. But copying a file between drives leaves a copy of the file on both drives. If you’d rather delete the original file, you can use OS X modifier keys to delete the file from the source drive simultaneously while copying to the new drive.
To delete the source file while it’s copying to another drive, simply hold down the Apple (Command) key while you’re dragging the file to the other drive. The file will copy to its new location while being removed from the source drive. This both saves you time and hard drive space, as you won’t have numerous copies of the same file hanging out on numerous hard drives.
As when deleting any file, be SURE you want to throw it away before you pull the trigger!