1.) Play through a LASO campaign mission and save/quit before you trigger the end of the mission.
2.) Go to the dashboard -> settings -> system -> storage -> (primary storage device) -> games and apps -> Halo Reach
3.) Scroll down to find your gamertag (11MB) which contains the checkpoint.
4.) Select and copy it to a secondary storage device to back up.
Keep track of the file’s date/time on a piece of paper and write a meaningful name (e.g., LNOS, POA, Package). If you plan on saving multiple campaign checkpoints onto your secondary storage device I suggest you create this written list otherwise you won’t be able to distinguish one file from another. Since multiple checkpoints won’t overwrite each other and they all have the same name (your gamertag) and 11MB file size the only way to distinguish one from another is by viewing its date/time. Keeping a record of those dates along with their description will make it much easier to find and restore your desired checkpoints.
To restore a saved checkpoint, first delete the 11MB file from your main storage device. Navigate to your secondary storage device -> games and apps -> Halo Reach. Locate the desired checkpoint that you wish to restore and copy it to your primary storage device. Launch Reach, resume/complete the campaign mission to instantly finish any weekly LASO, LD challenges, or other daily challenges that require completion with a certain combination of skulls.