Cassie would like to carry a backup with her wherever she goes, just in case. Leo says that portable hard drives and thumbdrives are dirt cheap these days, so she can not only do that, but she can put backups in several locations off site. Can she back up multiple computers to the same drive? Leo says perhaps, but recommends doing an image instead. She should image each drive and then save those images to the backup drive. That way she can restore each separate image as required.
Leo recommends Drive Snapshot.
Other options include:
Then she can restore the image as needed.