How to Back Up a Computer to an External Hard Drive

To back up a computer to an external hard drive, connect a drive with at least twice the capacity of your used data, turn on the built-in backup tool (File History or Backup and Restore on Windows, Time Machine on a Mac), pick the drive, and let the first full backup finish. After that, backups run automatically while the drive is plugged in. The whole setup takes about ten minutes of your attention, and the first copy may run for several hours depending on how much data you have.

This guide walks through the choice of drive, the right file system, the exact steps for Windows 10, Windows 11 and macOS, how to check that the backup actually works, and the habits that keep your data safe when a drive eventually fails. It also explains why one external drive is a start, not a finish.

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Step 0: pick the right drive before you start

The drive you choose decides how fast the first backup runs and how long the copy stays trustworthy. Use this table to match capacity and type to your situation.

The timing estimates assume a typical USB 3.0 hard drive that sustains around 100 to 150 MB/s for large files. At 120 MB/s, 1 TB takes about 2.3 hours (1,000,000 MB divided by 120 MB/s is roughly 8,300 seconds). Many small files are slower, because the drive has to seek. A portable SSD in a USB 3.2 Gen 2 port can sustain several hundred MB/s, which cuts the same job to under an hour.

Why twice your data

Backup software keeps older versions of changed files. If your drive is only slightly larger than your data, the oldest versions are removed quickly and you lose the ability to roll back to last month. Twice the used space gives a useful history. Three times is better if you work with large media files that change often.

Hard drive or SSD for backups

A hard drive gives the most capacity for the price and suits a backup that runs overnight. A portable SSD suits a laptop you carry, because it resists shocks and is much faster. If your backup is the only copy of irreplaceable photos, favor whichever drive you will actually keep connected, and add a second copy elsewhere. See the section below on the 3-2-1 rule.

Step 1: choose a file system that your computer can read

Most new external drives arrive formatted and ready to use. Check the format before you start, because the wrong file system limits which computers can read the drive and whether the backup tool will accept it.

If you use both a PC and a Mac, the cleanest approach is two partitions on one drive, or better, two separate drives. One partition formatted for Windows (NTFS) and one for Mac (APFS) will do. Do not rely on exFAT for the only copy of your backups, because it has no journal and handles unexpected disconnects poorly.

Important: formatting erases everything on the drive. If the drive already holds files you care about, copy them elsewhere first.

Back up a Windows 11 or Windows 10 PC with File History

File History is the simplest built-in tool. It saves copies of files in your Documents, Pictures, Music, Videos and Desktop folders, plus the offline OneDrive files on your PC, on a schedule you control. It does not copy programs or Windows itself. Use the system image option in the next section for that.

1. Connect the drive. Plug it into a USB port on the computer. Use a port on the computer itself rather than an unpowered hub.
2. Open File History. On Windows 11, open Control Panel, set View by to Large icons and choose File History. On Windows 10, go to Settings, then Update and Security, then Backup.
3. Select the drive. On Windows 10, choose Add a drive and pick your external drive. On Windows 11, File History picks the connected drive automatically, or lets you choose with Select drive.
4. Turn it on. Click Turn on. The first backup begins immediately.
5. Set the schedule. Open Advanced settings. The default is every hour, which suits most people. Set “Keep my backups” to a period such as 1 year, or to “Until space is needed” if you want the drive to fill gradually.
6. Choose folders. Under the folder exclusion options, remove folders you do not need, such as downloaded installers, and add any custom folders by placing them in a library.

Per Microsoft’s documentation, File History needs a drive formatted as NTFS or exFAT. If your drive is not detected, check its format in Disk Management.

When to use File History and when not to

• Good for: documents, photos and everyday files that change; recovering a previous version of a file; and quick restores after accidental deletion.
• Not for: full system recovery after a failed drive. It will not rebuild Windows or reinstall your applications.

Create a full system image with Backup and Restore

Windows 10 and Windows 11 still include the older “Backup and Restore (Windows 7)” tool, which can create a full system image of your drive, including Windows, programs and settings. A system image lets you recover the entire computer to a new drive after a failure.

1. Open Control Panel and select Backup and Restore (Windows 7).
2. Choose Create a system image on the left.
3. Select your external drive as the destination.
4. Confirm which drives to include. The system partition is selected automatically.
5. Click Start backup. Create a system repair disc when prompted if you do not have a recovery USB.

A system image is a snapshot in time. It does not update automatically, so create a fresh one after major changes such as a Windows version upgrade or a big software install. Many people keep File History running for daily protection and refresh the image every few months.

Back up a Mac with Time Machine

Time Machine is built into macOS and keeps hourly backups for the past 24 hours, daily backups for the past month and weekly backups until the drive fills. It backs up the whole Mac, including apps, settings and files.

1. Connect the drive. macOS may ask whether to use the drive for Time Machine. Choose Use as Backup Disk.
2. Or open System Settings manually. Go to General, then Time Machine, and click Add Backup Disk.
3. Select the drive. If the drive is not in a Mac-friendly format, macOS offers to erase it. Accept only if the drive has nothing you need.
4. Encrypt if the drive travels. Tick the encryption option for a laptop backup that leaves the house. Store the password somewhere safe, because there is no recovery if you lose it.
5. Exclude folders. In Options, add large folders you do not need, such as virtual machines or downloads.

The first Time Machine run is the slow one. After that, only changed files are copied, usually a few minutes per hour of activity.

Back up without built-in tools: drag and drop and third-party software

If you only need to copy a folder of photos or documents, you can drag it onto the drive. That is a copy, not a versioned backup, but it works for a quick safety net. For ongoing protection beyond the built-in tools, third-party backup software can add scheduling by file type, compression, encryption and the ability to back up to more than one destination. Whatever you choose, check that it can restore files on a different computer, since your current machine may be the thing that fails.

Check that your backup actually works

A backup you have never restored from is a hope, not a plan. Run these checks after the first backup completes and every few months afterwards.

1. Open the drive and look. Confirm your main folders appear and the size is sensible.
2. Restore one file. In File History, use Restore personal files and pick a document. In Time Machine, enter Time Machine and restore a file to the desktop. Open it.
3. Check the date. Confirm the latest backup is recent. A drive that quietly stopped months ago is more common than you would expect.
4. Check drive health. On Windows, review SMART status with the drive maker’s tool. On a Mac, Disk Utility’s First Aid and the SMART status line give a quick view.
5. Look for errors. A backup that reports skipped files may be missing exactly the files you care about, such as those open in another program.

The 3-2-1 rule: why one external drive is not enough

An external drive protects you from a failed internal drive, a stolen laptop or an accidental deletion. It does not protect you from fire, flood, theft of both devices together, or ransomware that encrypts every connected drive. The standard guidance is the 3-2-1 rule:

• 3 copies of your data: the original and two backups.
• 2 different media types, such as an internal drive and an external drive, or a drive and a NAS.
• 1 copy off-site, such as a second drive stored at work or a cloud backup.

A practical household setup is the computer itself, one external drive that stays plugged in for automatic backups, and a second external drive that you update monthly and keep at a relative’s home or in a safe-deposit box. Rotate the two drives every few weeks. If a cloud service fits your needs, it can serve as the off-site copy instead.

Protecting against ransomware

Ransomware encrypts files on every drive it can reach, including a permanently connected backup. Reduce the risk by keeping at least one backup disconnected between runs, enabling version history so older, unencrypted copies remain, and keeping your operating system and security software current. A drive that sits in a drawer cannot be encrypted by malware on your PC.

Common backup mistakes and how to avoid them

Drive health: when to replace your backup drive

Hard drives wear out. Many last five years or more, but there is no guarantee, and backup drives that run daily see the same wear as any other disk. Replace the drive sooner if you notice any of the following:

• Repeated disconnects, even with a different cable and port.
• Clicking, grinding or repeated spin-up sounds.
• Backups that suddenly take much longer than before.
• SMART warnings, reallocated sectors or pending sectors reported by the drive’s utility.
• Files that fail to restore or open with errors.

If a drive starts to fail, stop using it for new writes, copy its contents to a healthy drive, and consider a professional recovery service for data you cannot replace. Repeated power cycling and attempted repairs on a failing disk can turn a recoverable situation into a permanent loss.

Backup strategy by user type

Frequently Asked Questions

How big should an external hard drive be to back up my computer?

Choose a drive at least twice the amount of data you are using, and more if you want long version history. If your computer holds 500 GB of files, a 1 TB drive is the minimum and a 2 TB drive is comfortable. Larger drives are usually only a little more expensive per terabyte, so buying one size up is often the better choice.

Do I need to format a new external drive before backing up?

Usually not, because most drives ship formatted as exFAT or NTFS. Check whether the format suits your backup tool. Time Machine on a modern Mac prefers APFS, and File History needs NTFS or exFAT. If you reformat, remember that the process erases everything on the drive.

Can I use the same external drive for Windows and Mac backups?

Yes, but not safely on one shared partition. Create separate partitions, one for each operating system, or use two drives. The partition used by Time Machine must be formatted for macOS, and the partition used by File History must be formatted for Windows.

How often should my computer back up to the external drive?

Hourly is a sensible default for the built-in tools while the drive is connected. If you only connect the drive occasionally, plug it in at least once a week. Anything you cannot afford to lose should also be copied to a second location.

Is an external hard drive backup the same as cloud storage?

No. An external drive is under your control, fast to restore and has no subscription, but it can be stolen, damaged or lost. Cloud services protect against local disasters but depend on your connection and a recurring plan. Using both covers the weaknesses of each.

What should I do if the backup drive is not recognized?

Try a different cable and port, then check whether the drive appears in Disk Management on Windows or Disk Utility on a Mac. If the drive appears but shows as unallocated or uninitialized, do not format it until you have tried to recover the data, since formatting reduces the chance of recovery. If the drive makes unusual noises, power it off and consult a recovery professional.