macOS doesn’t expose all these keyboard modifiers, and new and old users alike may not know or remember them.
Dragging files around the Finder was life-changing in the 1980s after decades of entering commands in a terminal or shell that required precise syntax. Decades later, some of us may have forgotten a few file-dragging basics—or never discovered them, whether we’re an old hand at the Mac or a newcomer. It’s a good time for a refresher on what happens when you drag files in the Finder. If you hold down modifier keys when you drag files, you can perform different functions.
- Move a file to a new location on the same volume: a simple drag and drop without holding down any keys moves files from one location to another on the same volume.
- Copy a file from one volume to another volume: a simple drag and drop from one volume to a different volume creates a copy on the the destination volume.
- Move a file to a different volume: hold down the Command key and drag an item to a different volume. The file will no longer be on the original volume. Command-dragging within the same volume copies it.
- Copy a file on the same volume: hold the Option or Command key while dragging a file to a different location. You can also choose File > Duplicate or press Command-D to duplicate selected items. The Finder renames them to avoid filename collisions. Editing the duplicate doesn’t affect the original. Option-dragging to a different volume is the same as a simple drag and drop, it creates a copy of the file on the destination volume.
Read more at MacWorld.com