The default setting for the Finder in Apple OSX Lion 10.7 does not show any drives on the desktop. This may be helpful for beginning users not sure what do do when browsing a hard drive full of system files, programs, and user data.
However, when installing downloaded programs (DMG files) that open into virtual disk drives, this creates a problem of leaving these drives mounted in the operating system.
Also, for those on enterprise / business networks. People sometimes access a private network drive to use files. If the drive isn’t visible on the desktop, the user may forget to unmount/eject it. So, it’s left open for others to access.
To avoid this problem, from the Finder menu, choose Preferences and make sure the following boxes are selected.