Moving and Copying Disk Icons in Mac OS X Lion
Moving a file icon from one on-screen disk to another in Mac OS X Lion works the same way as moving an icon from one folder to another, with one notable exception: When you move a file from one disk to another, you automatically make a copy of it, leaving the original untouched and unmoved.
In OS X Lion if you want to move a file or folder completely from one disk to another, you have to delete that leftover original by dragging it to the Trash or by holding down the Command key when you drag it from one disk to the other.
You can’t remove a file from a read-only disc (such as a CD-R or DVD-R) or from a folder to which you don’t have write permission. But you should be able to move or delete files and folders from all other kinds of disks that you might encounter.
Copying the entire contents of any disk or volume (CD, DVD, or external hard drive, among others) to a new destination works a little differently:
Click the disk’s icon.
Hold down the Option key, and drag the disk icon onto any folder, any disk icon, or any open Finder window.
When the copy is completed, a folder bearing the same name as the copied disk appears in the destination folder or disk. The new folder contains each and every file that was on the disk of the same name.
Copying files in this way is handy when you want to grab all the files from a CD or DVD and put them on your hard drive.
If you don’t hold down the Option key when you drag a disk icon to another destination, your Mac creates an alias of the disk (that is, a link back to the original) instead of a copy of its contents. As you might expect, the alias will be almost worthless after you eject the disk; if you open it, it will ask you to insert the original disk.
If you like using the Duplicate command, note that you can’t use the Duplicate keyboard shortcut (Command+D) on a disk, although you can use it on a folder.