If the sad faced Mac icon appears after your disk starts spinning, the first two digits of the code number under the icon are OF, the next four digits will indicate the type of error. If you get a sad Mac with an OF code, restart the Macintosh with the Option and Command keys held down. This will rebuild the Desktop file (all Get Info. dialogs and file folder names will be lost). You might be able to fix a faulty startup disk by replacing its System file.