You must have got puzzled when the Mac system shows you message saying Your Startup Disk is full. You will notice that the Mac is taking much longer time to boot up and if you do not pay attention to it then a time will come where the Mac will not boot. The start up disk is the volume partition where the Mac OS X operating system has been installed and when you power on the computer all the files are loaded in the memory from there. When the start up is populated with junk and heavy files then the loading process gets difficult as the computer has to search for the relevant files. So you have to clean up the junk at regular interval to prevent Your Startup Disk is full issue. However if this as already occurred and the Mac is not starting then you may have to restore it to the factory setting. It will delete all the other files leaving just the files of the operating system. However in the process you may lose some of you important file which can be restored from time machine. If it fails then you can us e the Mac data recovery software which will restore only the selected files and resolve Your Startup Disk is full problem.