Hold ⌥ Option while you drag the file. A green + on the pointer means Finder will copy it and leave the original where it is.
Option-drag
- Start dragging the file to the folder where you want the copy.
- Hold ⌥ before you let go. A green + appears on the pointer.
- Drop it. The original stays put and a copy lands in the new folder.
Dragging to another disk, such as a USB drive, copies by default, without any key.
Copy and paste
- Select the file in Finder and press ⌘C.
- Open the other folder and press ⌘V.
Duplicate in the same folder
- Select the file and press ⌘D. Finder adds a copy with "copy" in its name.
With dragand
Drop a file on dragand's Hold zone and it keeps a copy for 24 hours, leaving the original where it was. Useful when you need the copy only for an upload or an email; it clears itself afterwards.