Moving Finder items

Drag to move a file elsewhere on your startup drive, and ⌘-drag to move to a mounted drive. Option-key drag to copy a file from one location to another on the startup drive, and just drag over a mounted drive to copy there.

This is just what I need to know. If only I knew which is my startup drive and what a mounted drive is. Can you tell me that?

It doesn't really matter. The key is what badge appears on the icon when you are about to drop the file.

If a green + badge appears when you are about to drop the file, then it will make a copy. If you want to move it instead, hold down the option key. If you want a copy, but no green + badge appears, hold down the option key to make it appear and create a copy.

The startup volume is the one that contains the OS. It is normally named Macintosh HD, but could be anything you want.

Any other drive that shows up in the Finder is a "mounted" drive.

They only copy if you're moving across volumes. It's a simple safety thing.