If "Save Recovery Session" under Tools menu doesn’t work then you could save the recovery session file by closing the software. Once you click on close (X) button then software displays a prompt to save the Recovery Session file. Select Yes and provide a location to save the Recovery Session file.
If the saved Recovery Session file fails to open, then the file might have got corrupted. The Recovery Session file gets corrupted when you disconnect the drive from your computer and connect it again to a different USB port. Connect the drive to the same USB port where it was initially connected and try to load the Recovery Session file. If the saved Recovery Session fails to open even after connecting the drive to the same USB port, the file might have got severely corrupted. In this case you need to re-scan the drive again to recover your lost files.