Target Validation ensures that the backup is made to the correct device.

Imagine you have an external Hard Drive which you use to backup and at a given moment you unplug it in order to plug in a USB stick of a colleague, just to make a quick copy of a file or photo.
Though, you do not want that meanwhile Yadis! copies files to that USB device.
So Yadis! will check that the device at the given drive letter is actually the device that you validated when you created the task.

The validation is done in two steps:
When you create a task, you choose a backup target.
The moment that you save the task and acknowledges that the specified target reflects the physical device you want to use, there is put a small file onto that device.
Then whenever a file (or number of files) needs to be copied, the target path is checked for that special file that was put during target validation.

In this way you are sure your files are saved on the correct physical device.