Moving file between disks in Mac OS X?
On macOS Mojave 10.14:

To move files to a different disk: Press and hold the Command key, then drag the files to the disk.

To keep an item in its original location and put a copy in a folder: Press and hold the Option key, then drag the item to the folder.

In older macOS releases, you could press the option key to change the behaviour


move instead of copy between different volumes
copy instead of move on the same volume