Disk Icons in OS X Mountain Lion
You should think of the disk icons that appear on the Mountain Lion Desktop (and/or in the sidebars of Finder windows) as though they were folders. That’s because your Mac sees disks as nothing but giant folders.
When you double-click one, its contents appear in a Finder window; to that extent, it works just like a folder. You can drag stuff in and out of a disk’s window, and you can manipulate the disk’s window in all the usual ways — again, just like a folder.
If you don’t see your hard disk icon(s) on the Desktop or in the Sidebar, open Finder Preferences (choose Finder→Preferences or press Command+, [comma]) and select the appropriate items in the General and Sidebar tabs.
Although (for all intents and purposes) disks are folders, disks do behave in unique ways sometimes.
Disks that need to be formatted before use
Brand-new disks sometimes need to be formatted — prepared to receive Macintosh files — before you can use them. When you connect an unformatted hard disk, your Mac usually pops up a dialog that asks what you want to do with the disk.
One option is usually to format (or initialize) the disk — that is, get it ready to record data. If you choose to format the disk, the Disk Utility program launches itself so you can format the disk from the Erase tab.
If you ever need to format or initialize a blank disk and don’t see the dialog, all you have to do is open Disk Utility manually (it’s in your Applications/Utilities folder) and use its Erase tab to format the disk.
How to move and copy between disks
Moving a file icon from one on-screen disk to another 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.
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.