How to Back Up a Computer to an External Hard Drive

Knowing how to back up a computer to an external hard drive is the cheapest insurance you can buy against a dead laptop, a failed drive, ransomware, or a spilled coffee. Both Windows and macOS have backup tools built in, and once they’re set up they mostly run themselves. This guide covers choosing and preparing a drive, the built-in tools on each system, how to check that your backup actually works, and the mistakes that leave people without a backup when they need one.

Choosing an external drive for backups

• Capacity: at least as large as the data you’re backing up, and ideally two or more times larger. Backup tools keep older versions of files, and those take space.
• Hard drive or SSD: a portable hard drive gives you the most space for the money. An external SSD is faster and handles bumps better, but costs more per terabyte. Either works.
• Use it only for backups. A drive that doubles as everyday storage is more likely to fill up, get carried around, and get lost or dropped.

Preparing the drive

New drives often come formatted as exFAT, which works on both Windows and Mac but isn’t ideal for either backup system.

• Windows: format the drive as NTFS for backups. System image backups require it.
• Mac: Time Machine will offer to erase and reformat the drive for you, using APFS on recent versions of macOS.

Formatting erases the drive. If it already has files on it, copy them somewhere else first. Our exFAT vs NTFS guide explains the differences between the formats.

How to back up a Windows computer to an external hard drive

Windows has several built-in options, and Microsoft has changed them over the years, so the exact menus depend on your version.

File History (Windows 10 and 11)

File History saves copies of your personal files (Documents, Pictures, Desktop, and other folders in your libraries) on a schedule, and keeps older versions so you can go back in time.

• Windows 10: Settings → Update & Security → Backup → Add a drive, then choose your external drive. Use More options to set how often it backs up, how long it keeps versions, and which folders it includes.
• Windows 11: open Control Panel → System and Security → File History, select the drive, and click Turn on.

To restore, open File History and choose Restore personal files.

Windows Backup app (Windows 11)

Windows 11 includes a Windows Backup app. As of this writing, it’s built around backing up folders, settings, and apps to OneDrive and your Microsoft account, rather than to an external drive. It’s useful alongside a drive backup, but it isn’t a replacement for one, and how much it can store depends on your OneDrive space.

System image (legacy)

A system image is a complete copy of the drive, including Windows, programs, and files, that can restore the whole computer at once.

1. Open Control Panel → System and Security → Backup and Restore (Windows 7).
2. Click Create a system image.
3. Choose On a hard disk and select your external drive.

Microsoft treats this as a legacy feature and no longer develops it, though it’s still present in Windows 10 and 11 as of this writing. It’s worth having, but don’t rely on it as your only backup.

How to back up a Mac to an external hard drive with Time Machine

Time Machine is built into every Mac and is the simplest backup on either platform.

1. Connect the external drive.
2. On macOS Ventura and later: Apple menu → System Settings → General → Time Machine → Add Backup Disk. On older versions: System Preferences → Time Machine → Select Backup Disk.
3. Choose the drive. Tick Encrypt Backup if you want it password-protected, and store that password somewhere safe.
4. Let the first backup finish. It can take hours.

While the drive is connected, Time Machine backs up automatically, keeping hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for older months, until the drive fills and the oldest are removed.

To restore, open the folder you need and choose Browse Time Machine Backups from the Time Machine menu, or use Migration Assistant to move everything to a new Mac.

Check that the backup actually works

A backup you’ve never tested is a guess. Every so often:

• Open the backup and find a recent file. Confirm it’s there and opens.
• Restore a test file to a different location and open it.
• Check the date of the last backup. File History and Time Machine both show it. If it’s weeks old, the drive hasn’t been connected or something has stopped working.
• Check the drive’s health occasionally. Our guide on how to check hard drive health shows you how.

Always eject the drive properly before unplugging it.

Follow the 3-2-1 rule

One external drive is a big improvement over nothing, but it shares many risks with your computer. A fire, flood, theft, or power surge can take both at once. The 3-2-1 backup rule is the standard answer:

• 3 copies of your data
• on 2 different types of storage
• with 1 copy kept off-site

In practice, that can mean your computer, an external backup drive at home, and either a cloud backup service or a second drive you keep at work or with a family member and swap regularly.

Don’t let the backup be the only copy

The most common backup mistake isn’t technical. People move files to an external drive to free up space on their computer, then think of that drive as their backup. It isn’t. If the files exist only on the external drive, that drive is now the original, and when it fails, which external drives do, nothing is left.

Keep the originals on the computer, or keep at least two copies on separate drives. If an external drive starts disconnecting on its own, clicking, or slowing down, copy the data somewhere else while you still can.

The bottom line

To back up a computer to an external hard drive, choose a drive with plenty of room, format it for your system, and turn on File History on Windows or Time Machine on a Mac. Add a system image on Windows if you want a full-machine copy. Test the backup now and then, keep a copy off-site, and never let an external drive hold the only copy of anything you care about.

If the external drive was the only copy and it has failed, our external hard drive recovery service recovers data from failed drives for one flat $1,200 price per single device, with free prepaid shipping, and you pay nothing unless the recovery succeeds.