If you need to stop a SyncToy run or preview, select Stop at the bottom of the screen. If you want to, you can resume the run or preview at the point you left off.
Stopping a run does not undo the actions that SyncToy has already taken to synchronize your folder pairs. Typically after you press the Stop button, some SyncToy actions will already have taken place and the synchronization will be only partially complete.
See also: