In Lion, whenever I drag and drop a file to or from an external drive to the local drive it always moves the file. If I'm not mistaken the default behavior is to copy the file right? Does anyone know why it might be doing this or know of a preference anywhere that I can set it to explicitly copy the files?
To copy files instead of move them, you can use menu items in the Edit menu, or their associated keyboard shortcuts. I am running Lion, and I have an external usb drive. If I select a file on this external drive and open the Finder's "Edit" menu, there's a "Copy" menu item with the shortcut Cmd-C.
Choosing that menu item, then selecting a folder on my internal hard drive, gives a menu item of "Paste" (again in the "Edit" menu) with the shortcut Cmd-V.
Also, dragging and dropping with the option key held down will copy a file rather than move it.