Mac disk utility could not validate source is a big problem and can lead to serious data loss situations from your Mac system. After usage for some time period the mac hard drive can get corrupted or start showing error or sluggish behavior. For dealing with that Mac is equipped with disk utility that can fix minor errors in the drive but sometimes you can get Mac disk utility could not validate source. This error depicts that there is a serious issue with your drive and it can not be fixed with the use of disk utility. What you can do now is to check for the error manually, it can be done by restarting the Mac system in the safe mode and then run the fsck command in the terminal that can fix the trouble. But if you are not skilled enough then you can simply format the drive and if it was the start up disk then you have to reinstall the Mac OS X. You can can get back the lost data if you have backup of it otherwise you can use the Mac data recovery software which will certainly recover all the data on Mac. You do not have to worry about Mac disk utility could not validate source anymore.