Backing up a laptop, such as a MacBook, MacBook Pro, or MacBook Air, is one of the trickiest problems in computing.

Wonderful though Time Machine is for an immobile system with plenty of fast external storage space, if your laptop seldom has access to a suitable external drive, it will inevitably go long between making proper backups.

Laptops are at the same time much more likely to suffer failure and data loss. They tend to run at higher temperatures, ageing their hardware components more rapidly. They can be subject to physical and mechanical trauma, and are all too easily stolen or misplaced.

If you only ever use your laptop when connected to capacious Internet connections, iCloud or another cloud service could be ideal, if relatively costly. But most mobile laptop users hardly ever have that opportunity.

Time Machine does offer a stopgap solution, by making local snapshots to your startup drive. For these to occur, you need to have Time Machine turned on, in its pane in System Preferences, and to be away from your normal Time Machine backup drive. Then every 24 hours after starting your laptop up, Time Machine will automatically make a local snapshot backup to your startup drive, and every week it will make an additional local snapshot.

If you discover that the current copy of a working document has become corrupted, or lost, you can then open the Time Machine app in the normal way. If you are away from your backup drive, backups which you can restore from are them marked with a bright red tick; those which are only accessible from your full backup drive will be shown with dimmed ticks, and their contents will appear blank.

These local snapshots are not full backups, but store those files which have changed. Time Machine will keep making them until there is only 20% of your startup drive free; it will then (just as it would on a regular backup) start removing the oldest snapshots to recover free storage space.

Local snapshot removal becomes more aggressive if free space falls to 10% or less (or less than 5 GB). If necessary, there will be only the latest snapshot stored, and Time Machine will stop creating new ones.

These local snapshots are not included in Finder displays of storage space used, but they are shown if you open the About This Mac dialog and select the Storage tab. Apple’s standard colour scheme shows all backups in purple: if you have such backups on your startup drive, then they will be local snapshots.

Local snapshots are a mixed blessing. If your drive suffers any significant failure, or is lost altogether, so will its local snapshots. Although better than nothing, you may decide that they are not worth the drive space that they occupy. You can then turn them off, using the Terminal command
sudo tmutil disablelocal
and entering your admin password at the prompt. You can enable them again using the complementary command
sudo tmutil enablelocal
likewise.

If you ever want to create an additional local snapshot, the command
tmutil snapshot
should do the job; it should not need to be run as root using the sudo command. You can also list all local snapshots with the command
tmutil listbackups

The following command should inform you of the path to your local snapshots folder:
tmutil machinedirectory

Recent versions of OS X, and compatible apps, also use versioning to store copies of documents which you are working on. One more efficient way of using limited storage on a laptop is to turn Time Machine local snapshots off, and to rely on saved versions of your active documents. This also occurs more frequently, at hourly intervals or more often. However, if you do that, you will be unable to restore from a proper backup until you return and connect to your full Time Machine backup.

As I wrote at the start, backing up laptops remains one of the trickiest problems.