Are you getting error 254 could not validate source on your Mac ? This is quite an annoying situation as when you are trying to mount a disk image on Mac system the Mac OS X shows this error. This image can be of installation file of any software or it can be backup file image. Whatever the case is the image file is very important to and it is not readable and showing error 254 could not validate source. The reason behind it can be that the image has been corrupted somehow. You need to find another copy of that image by downloading or getting a fresh disk image from the source. If the error 254 could not validate source shows up with various image files then it may be the case of OS corruption. You have to reinstall the Mac OS X to remove the corruption. In the meanwhile several other files will get removed from the system which you can get back from the backup. IF the backup does not work then you can use the Mac data recovery software to get back lost data after fixing error 254 could not validate source.