What happens when you back up to an external drive

When you back up your computer to an external hard drive, you are copying your files, folders, programs, and system settings to a separate device that sits outside your computer. If your computer fails, gets stolen, or is damaged, those files still exist on the external drive. You can then restore them to a new computer or to the same computer after repair.

The external drive connects to your computer through a USB cable (usually USB 3.0 or USB-C, depending on your computer's ports). The backup process can happen once, on a schedule you set, or continuously in the background. Most people use scheduled backups — weekly or monthly — so they do not have to remember to do it manually.

An external hard drive is different from cloud storage. The drive stays in your physical possession, which means no monthly fees and no dependence on internet speed. It also means the drive itself can fail or be lost, so many people use both: an external drive for fast, frequent backups and cloud storage as a second copy.

Key Takeaways

  • External hard drives connect via USB and hold a complete copy of your files, separate from your computer.
  • Windows computers use File History or System Image; Mac computers use Time Machine — both are built in and free.
  • A full backup of your entire system usually takes one to four hours the first time, then incremental backups take minutes.
  • After the first backup, set your backup software to run on a schedule so you do not have to remember to do it manually.
  • Test your backup by restoring a single file before you need it, so you know the process works when a real problem happens.

Choosing and preparing your external hard drive

An external hard drive needs to be large enough to hold your entire computer — usually 1 to 4 terabytes for a typical home computer. A 2 TB drive costs between $50 and $100 and holds roughly twice the amount of data most people accumulate. If you have a lot of video, photos, or music, go larger; if you mostly use documents and email, 1 TB may be enough.

When you first plug in a new external drive, your computer will recognize it as a storage device. On Windows, it appears in File Explorer. On Mac, it appears on the desktop and in Finder. Before you start backing up, format the drive so it is optimized for your computer's operating system. On Windows, right-click the drive, select Format, and choose NTFS as the file system. On Mac, open Disk Utility (in Applications > Utilities), select the drive, click Erase, and choose Mac OS Extended or APFS.

Keep the external drive plugged in during the first backup — this can take several hours depending on how much data you have. After that, you can unplug it and store it safely between scheduled backups. Some people keep their external drive at home; others keep a second copy at a different location in case of fire or theft.

Setting up backups on Windows using File History

File History is the built-in backup tool on Windows 10 and Windows 11. It backs up your Documents, Desktop, Downloads, Pictures, Music, and Videos folders — the places where most people keep their important files. It does not back up installed programs, but you can reinstall those from their original sources if needed.

To turn on File History, plug in your external drive, then open Settings (press Windows key + I). Go to System > Storage > Advanced storage options > Backup options. Click "Back up now" under File History. Windows will ask you to select the external drive as your backup location. Choose your external drive and click "Use this drive."

Once File History is running, it automatically backs up your files every hour by default. You can change this schedule by going back to the same Backup options screen and clicking "More options" under File History. You can set it to back up every 10 minutes, hourly, daily, or weekly. Most people choose daily or weekly so the backup does not slow down their computer during work hours.

File History keeps multiple versions of your files, so if you accidentally delete something or overwrite it, you can restore an older version. Right-click any folder, select "Restore previous versions," and choose the date you want to restore from.

Setting up backups on Mac using Time Machine

Time Machine is the built-in backup tool on macOS. It backs up everything on your Mac — your files, applications, system settings, and email — so you can restore your entire computer if something goes wrong.

To set up Time Machine, plug in your external drive. A dialog box usually appears asking if you want to use the drive for Time Machine backups. Click "Use as Backup Disk." If the dialog does not appear, open System Preferences (or System Settings on newer Macs), go to Time Machine, and click "Select Backup Disk." Choose your external drive and click "Use Disk."

Time Machine will begin backing up when ready. The first backup takes several hours and backs up your entire system. After that, Time Machine runs automatically every hour and backs up only the files that have changed since the last backup — these incremental backups usually take just a few minutes.

To restore a file, open Time Machine, navigate to the date when the file existed, and click Restore. To restore your entire system, restart your Mac while holding Command + R to enter Recovery Mode, then select "Restore from Time Machine Backup."

What to back up if you want more control

If you prefer to choose exactly which folders to back up instead of using File History or Time Machine, you can use third-party software like Macrium Reflect (Windows), Acronis True Image (Windows and Mac), or Backblaze (Windows and Mac). These tools let you create a system image — an exact copy of your entire hard drive that you can restore to a new computer if your current one fails completely.

A system image is more powerful than File History because it includes your operating system, all your programs, and all your settings. If your hard drive dies, you can restore the image to a new drive and your computer will work exactly as it did before. The trade-off is that system images take up more space and take longer to create and restore.

Most of these tools offer a free version with basic features. Macrium Reflect Free, for example, lets you create one system image and schedule automatic backups. Paid versions add features like incremental backups (backing up only what changed) and the ability to restore to different hardware.

How long backups take and what to expect

Your first backup will take the longest — usually one to four hours depending on how much data you have and how fast your external drive is. A USB 3.0 drive can transfer about 100 to 150 megabytes per second, so a 500 GB backup takes roughly one hour. USB 2.0 drives are much slower and should be avoided for large backups.

After the first backup, scheduled backups are much faster because your backup software only copies files that are new or changed. If you back up daily, each backup might take 5 to 30 minutes depending on how much you use your computer. If you back up weekly, it might take 15 minutes to an hour.

During a backup, your computer will run slightly slower because the hard drive is busy copying files. You can use your computer normally while a backup is happening, but large tasks like video editing or gaming may feel sluggish. Most people schedule backups for times when they are not using their computer — overnight, during lunch, or on weekends.

Testing your backup before you need it

A backup is only useful if it actually works. Before you rely on it, test it by restoring a single file. On Windows with File History, right-click a folder, select "Restore previous versions," choose a date, and restore one file to a test location. On Mac with Time Machine, open Time Machine, navigate to a past date, and restore a file you do not need.

If the file restores successfully, your backup is working. If it does not, check that your external drive is still connected and that your backup software is still running. If you use a third-party tool like Macrium Reflect, test restoring a system image to a USB drive or external drive to make sure the image is valid before you need it in an emergency.

Also check your external drive occasionally to make sure it is still working. External drives can fail without warning, especially if they are old or have been dropped. If you hear clicking sounds or your computer does not recognize the drive, stop using it when ready and consider it failed. This is why many people keep two external drives and rotate between them, or use both an external drive and cloud storage as backup.

Frequently Asked Questions

Can I use the same external drive for backups and for storing other files?

Yes, but it is not ideal. If you use half the drive for backups and half for other files, you reduce the space available for backups. More importantly, if the drive fails, you lose both your backups and your other files. Most people use a dedicated external drive for backups only, or use a much larger drive (4 TB or more) so backups have plenty of room.

What if my external drive is full?

File History and Time Machine automatically delete old backups to make room for new ones, keeping the most recent backups and deleting the oldest. You can also manually delete old backup versions if you need space. If you run out of space frequently, your drive is too small — consider upgrading to a larger one.

Do I need to encrypt my external backup drive?

If your backup contains sensitive information like financial records, passwords, or personal documents, encryption is a good idea. On Windows, right-click the drive and select "Turn on BitLocker" (available on Windows Pro and higher). On Mac, open Disk Utility, select the drive, and click "Encrypt." You will need to enter a password each time you connect the drive, but your files will be protected if the drive is lost or stolen.

Can I back up to an external drive over Wi-Fi instead of USB?

Yes, if your external drive is connected to a network-attached storage (NAS) device or a wireless external drive. These are more expensive than USB drives but let you back up without plugging in a cable. USB drives are faster and cheaper for most home users, but wireless backups are convenient if you want automatic backups without managing cables.

What should I do if my external drive fails?

If your external drive stops working, do not try to repair it yourself — you risk making data recovery impossible. If the drive is still under warranty, contact the manufacturer. If you need the data urgently, a professional data recovery service can sometimes recover files, but it is expensive. This is why keeping a second backup (either a second external drive or cloud storage) is important.