How to Back Up Your Computer and Phone: The 3-2-1 Rule Step by Step

Losing your photos, documents or work files can happen in seconds through a dead hard drive, a stolen phone, a spilled drink or a ransomware attack. The simplest protection is the 3-2-1 backup rule: keep three copies of your data, on two different types of storage, with one copy kept off-site. This step-by-step guide shows how to back up an iPhone, Android phone, Windows PC and Mac, which tools are free, how much storage you need and how to check your backups actually work.

What Is the 3-2-1 Backup Rule?

The 3-2-1 rule is the most widely recommended backup strategy and is promoted by the US Cybersecurity and Infrastructure Security Agency (CISA). It means:

• 3 copies of your data: the original plus two backups
• 2 different storage types: for example, your computer’s internal drive and an external hard drive, or a drive and the cloud
• 1 copy off-site: stored somewhere else, such as a cloud service or a drive kept at a relative’s home or office, so a fire, flood or burglary cannot destroy everything

Many experts now add a further step: keep at least one backup offline or immutable, meaning it cannot be changed or deleted by ransomware that infects your computer.

Why Is Syncing Not the Same as Backing Up?

Services such as iCloud Drive, Google Drive, OneDrive and Dropbox sync files across devices. That is convenient, but if you delete or corrupt a file on one device, the change can quickly spread to the others. A true backup keeps separate copies, ideally with version history, so you can restore a file as it was last week. Most cloud services keep deleted files and old versions for a limited period, often 30 days, so check your settings.

How Do You Back Up an iPhone?

• iCloud Backup: go to Settings, tap your name, then iCloud, then iCloud Backup, and switch it on. Your iPhone backs up automatically when it is locked, charging and on Wi-Fi. Apple gives 5 GB free, which fills quickly, so most people need an iCloud+ plan
• Computer backup: connect the iPhone to a Mac (using Finder) or a Windows PC (using the Apple Devices app or iTunes) and choose to back up all data to the computer. Tick “Encrypt local backup” to include saved passwords and health data
• Photos: iCloud Photos syncs your library, but also export or back up photos separately so they are not only in one place

How Do You Back Up an Android Phone?

• Google One backup: open Settings, then Google or System, then Backup, and turn on backup by Google One. It saves apps, call history, contacts, settings and SMS messages, and uses your Google account storage, which includes 15 GB free shared across Gmail, Drive and Photos
• Google Photos: turn on backup in the Photos app to store pictures and videos in your Google account
• Manufacturer tools: Samsung and some other brands offer their own backup and transfer apps
• Computer copy: connect your phone by USB and copy the DCIM folder of photos and videos to your computer or external drive

How Do You Back Up a Windows PC?

• Windows Backup app (Windows 11): backs up folders, settings, apps and credentials to your Microsoft account and OneDrive. Free OneDrive storage is 5 GB, so large libraries need a Microsoft 365 plan
• File History: connect an external drive, then search for File History in the Control Panel and turn it on to keep regular versioned copies of your files
• System image: third-party tools can copy the entire drive, so you can restore everything after a failure

If you are still running Windows 10, back up before upgrading or buying a new PC. Our guide to Windows 10 Extended Security Updates explains your options.

How Do You Back Up a Mac?

• Time Machine: connect an external drive, open System Settings, then General, then Time Machine, and add the drive. It keeps hourly backups for the past 24 hours, daily backups for the past month and weekly backups for all previous months until the drive is full
• iCloud Drive: syncs Desktop and Documents folders, but is not a substitute for Time Machine
• Cloud backup service: add an off-site backup with a service that continuously copies your Mac to the cloud

What Storage Should You Buy?

Use these tips when choosing backup storage:

• External drive size: pick a drive at least twice the size of the data you want to back up, so it can hold several versions
• Hard drive vs SSD: traditional hard drives offer the most space for the money; SSDs are faster and tougher but cost more per terabyte
• Cloud backup: dedicated services back up a whole computer for a monthly or annual fee and keep your copy off-site automatically
• Network storage (NAS): a home storage device suits families or small businesses backing up several computers

Black Friday is often a good time to buy drives; see our roundup of Black Friday tech deals.

How Do You Protect Backups From Hackers and Ransomware?

• Disconnect external drives after each backup, so ransomware cannot reach them
• Turn on two-factor authentication for cloud accounts; our guide to passwords, passkeys and 2FA explains how
• Encrypt backups that contain sensitive data
• Beware of phishing: many ransomware attacks start with a malicious email, so learn how to spot a phishing email
• Keep security software and updates current; see do you need antivirus?

What Should You Back Up First?

If you are starting from scratch, prioritise the data that would be hardest to replace:

• Photos and videos: usually the most precious and the largest files
• Personal documents: tax returns, IDs, insurance papers, medical records and contracts. Store sensitive files encrypted
• Work and study files: projects, theses and client work
• Passwords and two-factor recovery codes: export or save them securely in a password manager, which is itself backed up
• Messages: WhatsApp, Signal and other apps have their own backup settings, so turn them on separately

Apps, operating systems and most music or films bought from online stores can usually be downloaded again, so they are a lower priority.

How Often Should You Back Up and Test It?

Back up as often as you could bear to lose data. For most people, automatic daily backups to the cloud plus a weekly or monthly copy to an external drive is a good balance. Small businesses with constantly changing files may need hourly backups.

A backup you have never tested may not work. Every few months, try restoring a few files or photos to make sure they open correctly. Before selling or trading in a phone or computer, check you have a recent backup, then erase the device. World Backup Day on 31 March is a handy annual reminder.

The Bottom Line

Follow the 3-2-1 rule: keep three copies of your data on two kinds of storage with one off-site. Turn on automatic cloud backup for your phone, use Time Machine or File History with an external drive for your computer, keep one copy offline and test restores regularly. An hour of setup now can save years of irreplaceable photos and files.

Featured image: Woman working on her laptop. External hard drive plugged into the laptop, by Shixart1985 / Wikimedia Commons, CC BY 2.0. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• CISA (US-CERT) — Data Backup Options (accessed 2 November 2026).
• Apple Support — How to back up your iPhone, iPad and iPod touch (accessed 2 November 2026).
• Apple Support — Back up your Mac with Time Machine (accessed 2 November 2026).
• Google — Back up or restore data on your Android device (accessed 2 November 2026).
• Microsoft Support — Back up your Windows PC (accessed 2 November 2026).
• CISA — Stop Ransomware guide (accessed 2 November 2026).