A couple things you can check.
1. Make sure your user account, the drive the games are located on and the games themselves still have permission to write to the save location.
2. The drive containing the save location may be full
EDIT: Just had a thought, if you are bringing games to work/school where you would normally not be allowed to play said games and had found a way to circumvent the restrictions by playing off of an external drive you may be out of luck - they're on to you 😉