Is OSX prohibitory sign just annoying and you are looking for some solution to resolve this issue. Your Mac might display some warning sing while start up and 'prohibitory sign' is such a common alert which looks as a deep gray circle having a diagonal line in the middle along with a light gray screen. OSX prohibitory sign is the indication of something wrong happen on your system that need to be sorted out immediately. It represents the sad Mac icon and chimes of death during pre booting that makes system unable to boot. This error shows that system is able to find boot-able Mac installation but some problem in it. OSX prohibitory sign error may appear when there is important component missing during the installation of OS X. If you use multiple boot-able installation on Mac OS X then there is chance of getting this error. You will be able to fix OSX prohibitory sign by replacing mach_kernel file where you need to copy it from DVD to attached hard disk using command sudo cp /Volumes/"DVD_MEDIA_NAME"/mach_kernel /Volumes/"FIREWIRE_HD". After that the trouble still exist then format the hard disk and reinstall OS X which may turn to data loss, but no need to panic if there is lack of updated backup then Mac data recovery software gives rid from file loss without any hassle. It is able to restore the lost, corrupted data from Mac efficiently.