How do backups recover lost files?

1. Ana keeps her project in a folder: a story, a photo, a song and a drawing. One wrong tap, and the photo is deleted. Is it gone forever?
2. Luckily, Ana set up a backup days ago. A backup program copied every file in the folder onto a separate drive. That spare copy is called a backup.
3. The next day Ana adds to her story. The backup program checks which files changed and copies only those. That is much quicker than copying everything again.
4. The program does not throw the old backups away. It keeps a row of them, one for each day, like photos of the folder. Each one is called a version.
5. Back to today, and the missing photo. Ana picks the backup from yesterday, when the photo was still there, and the program copies it back. This is called restoring.
6. A drive on the same desk could be lost together with the laptop. So careful people keep one more backup far away, in another building or in the cloud.

Tap each part of Ana's backup, from her folder to the faraway copy.

• Your files. These are the working files: the ones Ana opens and changes. They live on the laptop's own drive, and that is the copy most likely to have an accident.
• Schedule. People forget to back up, so the program follows a schedule, such as every night. An automatic backup is one you never have to remember.
• Backup drive. The backup lives on a different drive from the working files. If the laptop's own drive breaks, the copies on this one are still fine.
• Versions. Each card is a version: what the folder looked like on one day. Keeping several lets you go back to before a mistake, even if you notice it days later.
• Changed file. The bright file is the only one that changed since the last backup, so it is the only one that was copied. The program remembers that the others are the same as before.
• Restore. Restoring copies files from a backup back to where you work. You can bring back a whole folder, or just the one file you need.
• Faraway copy. A second backup is kept somewhere else, such as a relative's home or a cloud service. If something happens to the whole room, this copy survives.

Change or delete files in the pretend folder, make backups on different days, then tap a backup card to restore it. What comes back, and what does not?

Experiment: You back up on Monday. On Tuesday you add to your story but do not back up. Then you restore Monday's backup. What happens to Tuesday's writing?

1. Tap the backup drive to back up today.
2. Press Next day, then tap the story to change it.
3. Do not back up. Tap the earlier backup card to restore it.
4. Look at the story. Which version is it?

What we found: Tuesday's writing is gone. A backup only holds what the folder looked like when the backup was made. Restoring it puts the folder back to that moment, so anything newer disappears. That is why backups should be made often.

Turn on JavaScript to try it yourself.

Someone scribbled on the drawing on Thursday. Get the good drawing back without losing the song Ana made on Thursday.

Everything on a computer is stored on a drive, and drives do not last forever. Files also vanish because of wrong taps, spilled drinks and lost laptops. A backup is a spare copy of your files kept on a different drive, so that one accident cannot destroy both.

A backup program does the copying for you, usually every day. The first time, it copies everything. After that it only copies the files that changed, which is much faster. It keeps the older copies too, each marked with its date. These dated copies are called versions.

Versions matter because you do not always notice a problem at once. If your drawing was spoiled on Thursday and you only see it on Saturday, the newest backups already contain the spoiled drawing. You need a version from before Thursday. Getting a file back from a backup is called restoring.

A backup can only give back what it holds. Work you did after the last backup is not in it. And a backup drive sitting next to the computer can be lost in the same accident, so many people keep one more copy far away. The game save point idea has a limit too: a backup does not undo things by itself. You must choose a version and restore it.

Key words

Backup

A spare copy of files, made at a known time and kept on separate storage.

Restore

To copy a file back from a backup to where you work.

Version

A backup from one particular day: what your files looked like then.

Full backup

A backup that copies every file, whether it changed or not.

Incremental backup

A backup that copies only the files that changed since last time.

Off-site copy

A backup kept in a different building, so one disaster cannot destroy every copy.

Check your thinking

1. What is a backup?

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   A spare copy of your files kept in another place. The copy must be somewhere else, so one accident cannot destroy both.

2. Your drawing was spoiled on Thursday. Which backup should you restore it from?

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   The newest one made before Thursday. Backups made after the damage hold the spoiled drawing. The newest earlier one has the most of your work.

3. You restore last Monday's backup of your whole folder. What happens to a file you created on Wednesday?

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   It disappears, because Monday's backup never had it. A whole-folder restore puts the folder back to how it was on Monday.

4. Why keep one backup in a different building?

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   So a fire, flood or thief cannot take the computer and every copy at once. One disaster can destroy everything in one room. A copy somewhere else survives it.

5. A folder holds 20 megabytes. Each day 2 megabytes of it change. About how much space do 7 daily versions need as incremental backups, if the first is a full copy?

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   About 32 megabytes. One full copy of 20, plus six increments of 2, makes 32. Seven full copies would need 140.

6. Why does a backup program copy only the files that changed?

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   It is faster and uses less space, and the other files are already saved. Files that did not change are already in an earlier backup.