Using Disks in Mac OS X Lion
Think of the disk icons that appear on the Desktop (and/or in the sidebars of Finder windows) in Mac OS X Lion as though they were folders. That’s because OS X Lion 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 documents and applications 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 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.
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.