Could not validate source 254 is a frequently occurring error on Mac operating system. Computer user utilizes Mac for storing massive amount of data like photos,emails,videos, files etc. Sometimes,there are some errors in mac OS which prohibit users to access the significant data."Could not validate source 254" is an example of such error, that hinders any task and ends the entire process. it is encountered when you try to restore the thumb drive or an external USB drive in Mac. Another cause of its occurring is because of the reason of the corrupted or destroyed ISO or the running of false file system.
It is required to take direct action to solve the could not validate source 254 error. if this error is not troubleshooted soon, it may cause data loss and system crash. This error message indicates that ISO has been damaged and the Mac OS is not able to write on the thumb drive. You will have to create a new ISO and remove the old one,on the condition that the source file is present of the created ISO. In the absence of it ,you may save the image of corrupted ISO. Disk Utility tool may be used to fix this error. It is advised to use Mac data recovery software to recover inaccessible data due to could not validate source 254 error.