Steam randomly validating games
Validation works similar to Windows' sfc /scannow command.
It will check the integrity of game files to make sure they are in working order.
It’s probably the easiest way to download, install, and play a full-retail video game and that has made the software so popular. Users have began to see that the games they already downloaded and installed require further downloads.
Games may appear deleted when Steam is no longer able to recognize the installation files.
Sadly, there is no workaround if the files are corrupted because they will need to be replaced.
Note: Sometimes this process can be unsuccessful because the appmanifest file for your game is nowhere to be found.
Sometimes that works and fixes the problem, but most of the time Verifying the data just means re-downloading the entire game.
Most people tell you to just go ahead and re-download the entire game because they think it is easier, even though the game is already on your system; How frustrating!
Do not run this process for multiple games at the same time.
Steam is a gaming platform targeted almost exclusively at PC users across various operating systems such as Linux, Mac OS X and Windows.