The Desktop file is an invisible file found in the main level of your
hard disk. It is the file that keeps track of all the documents and
applications that are on your disk as well as associations between the
files and programs. System 7.0 and later versions use the invisible
files named Desktop DB and Desktop DF to store the desktop information.
The Desktop File also manages all icons on a particular hard disk, zip
disk or floppy disk. This is also sometimes referred to as Desktop Database.
When to Rebuild the Desktop File?
Occasionally, the Desktop file may become too large or may become damaged
and unusable. According to Apple, it is generally a good idea to rebuild
your Desktop file once a month or so.
Symptoms of a damaged Desktop File
The computer
reports that the application that created the document cannot be found
when double-clicking a document. When you
insert a new or customized icon, the Desktop file may not load it or
may load a previous version of the icon. To have the special icon appear
on the desktop, the Desktop file must be rebuilt to update the Desktop
file and register the icon. Icons on
your desktop appear as generic document or generic application icons:
Generic icons:
Symptoms of a large Desktop File
If the Desktop file becomes too large, the computer may have difficulty
reading it efficiently and speedily. This can cause the Finder to access
files very slowly. Rebuilding the Desktop file can clean up old information
and speed up Finder access.
Rebuilding the Desktop File procedure
In
order to rebuild your Desktop File you need to restart your Mac
while pressing the Command and Option keys. However, because some
startup files may interrupt the desktop rebuilding process which
will result in totally garbled desktop it is recommended to reboot
your Mac with Extensions off. The procedure of rebooting with Extensions
off is described here.
Of course, turning the Extensions off and rebuilding the desktop
needs to be done during one startup cycle.
Here is the step-by-step description of the procedure:
First,
reboot your Mac... Right
after the startup chime press and hold the SHIFT key until you see
a message "Extensions off"... When
you see that message release the SHIFT key and immediately... Press
and hold the Command (Apple) and Option keys. Keep
holding them until you see a dialog box with the message, "Are you
sure you want to rebuild the Desktop file on the disk "your disk"?,"
Then
release the keys and click OK.
The progress bar will show and the process of rebuilding the Desktop
File will begin. It may take several minutes depending in the size
and speed of your hard disk and the number of files on it. If you
have more than one disk or partition you will be asked to rebuild
the desktop on each one of them. Click OK each time and let it finish.
Once it's finished and your computer finished booting up you will
need to restart it again to have it boot into normal working mode.
Remember that you have turned the Extension off!
The computer
reports that the application that created the document cannot be found
when double-clicking a document. 
