What happens when you back up to an external drive

When you back up your computer to an external hard drive, you copy your files, folders, and system settings to a separate device that you can store safely away from your computer. If your computer fails, gets stolen, or is damaged, you can restore everything from that external drive—your documents, photos, emails, and programs will still exist.

The backup process works differently depending on your operating system. Windows computers use built-in tools like File History or System Image, while Mac computers use Time Machine. Both create copies automatically on a schedule you set, so you do not have to remember to do it manually each time.

An external hard drive is straightforward a storage device you plug in via USB cable. It holds a complete copy of your data separate from your computer, which is why it protects you if something goes wrong with your main machine.

Key Takeaways

  • External hard drives connect to your computer via USB and store a complete copy of your files, protecting you if your computer fails or is damaged.
  • Windows computers use File History for regular backups or System Image for a complete copy of everything, while Mac computers use Time Machine.
  • You should back up at least once a month, though weekly or automatic daily backups catch more recent changes.
  • Before you start, check that your external drive has enough space—it should be at least as large as the amount of data on your computer.
  • Keep your external drive unplugged and stored in a safe place when you are not actively backing up, so it stays protected from power surges and theft.

Choosing and preparing your external hard drive

An external hard drive needs to be large enough to hold all your data. Check how much space your computer is using by opening File Explorer (Windows) or Finder (Mac), right-clicking your main drive, and selecting Properties or Get Info. If your computer shows 500 GB of data, buy an external drive with at least 1 TB (1,000 GB) of space—the extra room lets your backup software keep multiple versions of your files so you can recover older versions if needed.

When you first plug in a new external drive, your computer may ask you to format it. Formatting prepares the drive to receive data. On Windows, right-click the drive in File Explorer, select Format, choose NTFS as the file system, and click Start. On Mac, open Disk Utility (search for it in Spotlight), select the drive, click Erase, choose Mac OS Extended as the format, and confirm. This erases anything already on the drive, so only do this once, on a brand-new drive.

Give your external drive a clear name like "Computer Backup" or "Home Backup 2024" so you know what it contains. After formatting, safely eject the drive by right-clicking it and selecting Eject (Windows) or dragging it to the Trash (Mac), then unplug it.

Setting up automatic backups on Windows using File History

File History is the easiest backup method for Windows. Plug in your external drive, then open Settings by pressing Windows key + I. Go to System, then select Backup on the left side. Under Backup options, click Back up now, and Windows will ask you to select a drive. Choose your external hard drive from the list.

After the first backup finishes, File History will automatically back up your files every hour by default. You can change this schedule by going back to Settings > System > Backup, scrolling down to Advanced options, and selecting how often you want backups to run. Most people choose daily or weekly backups to balance protection with the time the process takes.

File History backs up your Documents, Desktop, Downloads, Pictures, Music, and Videos folders, plus your email and browser settings. It does not back up installed programs, but you can reinstall those from the internet if needed. If you want to back up everything including programs and Windows itself, use System Image instead (see the next section).

Creating a complete system image on Windows

A System Image is a complete copy of your entire computer—every file, every program, every setting. If your computer stops working, you can restore from the image and your computer will be exactly as it was. This takes longer than File History and uses more space, but it is the most thorough protection.

Plug in your external drive. Open Control Panel, search for "backup," and click Backup and Restore (Windows 7). On the left side, click Create a system image. Windows will ask you where to save it; select your external hard drive. Choose which drives to include (usually just your main C: drive), then click Next and Start backup. The process can take 30 minutes to several hours depending on how much data you have.

Once the image is created, Windows will ask if you want to create a system repair disc. You can skip this if you have another computer available, but creating one gives you a way to start your computer from the disc if your main drive fails completely. Store the repair disc in a safe place separate from your external drive.

Setting up automatic backups on Mac using Time Machine

Time Machine is Mac's built-in backup tool and works similarly to Windows File History. Plug in your external drive, then open System Settings (or System Preferences on older Macs). Search for Time Machine and click it. Click Select Backup Disk, choose your external drive, and click Use Disk.

Time Machine will start backing up when ready and will repeat automatically every hour. You can change the backup frequency by opening Time Machine settings again, though hourly backups are usually fine. Time Machine keeps hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than that, so you can recover files from any point in time.

Unlike Windows System Image, Time Machine does not back up installed programs—you will need to reinstall those from the App Store or the original installer if you need to restore. However, it backs up all your personal files, settings, and system configuration, which is what most people need.

What to do after your first backup completes

After your initial backup finishes, check that it actually worked. On Windows, go to Settings > System > Backup and look for the date and time of your last backup. On Mac, click the Time Machine icon in the menu bar and verify that the most recent backup shows today's date. If the backup shows an error, check that your external drive has enough free space and that it is still plugged in.

Unplug your external drive and store it in a safe, cool, dry place—ideally somewhere separate from your computer, like a drawer in another room or a safe. This protects it from theft, fire, or power surges that could damage both your computer and the drive at the same time. You do not need to keep it plugged in constantly; backups will resume automatically the next time you plug it in.

Check your backup once every few months by plugging in the drive and verifying the backup date is recent. If you have not backed up in more than a month, plug in the drive and let the backup run to completion. Most backup failures happen silently—the software stops working but does not alert you—so a quick monthly check prevents discovering too late that your backups have not been running.

Common mistakes that leave you unprotected

The most common mistake is leaving your external drive plugged in all the time. If your computer gets a virus or ransomware infection, the malware can spread to the external drive and corrupt your backup. Unplug the drive after each backup and store it away. Plug it in only when you are actively backing up or restoring files.

Another mistake is buying a drive that is too small. If your external drive is the same size as your computer's storage, you will run out of space after the first backup and subsequent backups will fail. Your external drive should be at least 1.5 times the size of your computer's data.

People also forget to check whether backups are actually running. Backup software can fail silently—the software stops working but does not send you a warning. Check your backup status once a month by looking at the date of the last successful backup. If the date is more than a month old, something has gone wrong and you need to troubleshoot it.

Finally, do not rely on only one backup method or one external drive. If your external drive fails, you have lost everything. Consider keeping two external drives and rotating them, or using both File History and a System Image on the same drive for extra protection.

Frequently Asked Questions

How long does a backup take?

Your first backup usually takes 1 to 4 hours depending on how much data you have and how fast your external drive is. Subsequent backups are much faster—usually 15 to 30 minutes—because the backup software only copies files that have changed since the last backup. Backups run in the background, so you can use your computer while it is happening.

What if my external drive stops working?

If your external drive fails, you have lost that backup. This is why keeping two external drives and rotating them is a good idea—if one fails, you still have a recent backup on the other. You can also consider cloud backup services as a second layer of protection, though those typically cost money per month.

Can I back up to a USB flash drive instead of an external hard drive?

USB flash drives are too small and too slow for regular backups of an entire computer. They work for backing up a single folder of important documents, but not for a complete system backup. External hard drives are much cheaper per gigabyte and much faster, making them the right tool for this job.

Do I need to back up if I use cloud storage like OneDrive or Google Drive?

Cloud storage backs up only the files you put in those folders, not your entire computer, programs, or system settings. If your hard drive fails, you would lose everything not in cloud storage. An external hard drive backup is still necessary for complete protection. You can use both—cloud storage for important documents and an external drive for everything else.

What should I do if I need to restore files from my backup?

On Windows with File History, plug in your external drive, open Settings > System > Backup, scroll down, and click Restore files from a backup. Select the date you want to restore from and choose which files to recover. On Mac with Time Machine, click the Time Machine icon in the menu bar, browse to the date you want, select the files, and click Restore. If you need to restore your entire computer from a System Image, you will need to start from a repair disc or recovery drive.