You can save your game anywhere you want so it really depends on where you personally decided to save it. You said not to guess so no one can respond as guessing is the best we can do. I am fairly sure that by default Unreal puts games in the Documents folder. Not sure what you mean by “Rebuild my system”. If you are re-installing windows then I believe you can keep all files intact during the new windows install (all data will go to Windows.old after install). If you mean you are actually re-building the laptop then in that case you might want to take the drive and put it in an external case and just pull from it that way.