If your source and destination are on different disks or even on different partitions on the same disk, then dragging and dropping will copy. It is only if source and destination are on the same partition that drag and drop will move the file. At least that is what I have found.

If you drag with your right mouse button, when you drop the file a menu appears which gives you the option to copy, move or create a shortcut at the new location. That is what I use if I want to move files between disks or partitions.

Geoff