What an external backup drive does and why you need one

An external backup drive is a hard drive you connect to your Mac through a USB or Thunderbolt cable. It stores a copy of your files, photos, documents, and system settings so that if your Mac's internal drive fails, gets stolen, or is damaged, you still have your data. Most Macs do not automatically back up to an external drive — you have to set it up yourself.

The two main ways to back up are Time Machine, which is built into macOS and creates hourly snapshots you can restore from, and manual copying, where you drag folders to the external drive yourself. Time Machine is faster to set up and restores more reliably, but manual copying gives you more control over what gets backed up.

Without a backup, a single hardware failure or accident can mean permanent loss of years of photos, work files, and personal documents. An external drive costs between $50 and $200 depending on storage size and is the most straightforward insurance against that loss.

Key Takeaways

  • Time Machine, built into macOS, automatically backs up your entire Mac to an external drive every hour once you plug it in and format it.
  • You must format the external drive as Mac OS Extended or APFS before Time Machine will recognize it — Windows format will not work.
  • The external drive should be at least as large as your Mac's internal drive, and ideally larger, so Time Machine can keep multiple backup versions.
  • After the first backup completes, Time Machine runs in the background and requires no action from you as long as the drive stays connected or nearby.
  • If you do not want automatic backups, you can manually copy folders to the external drive using Finder, though you will have to remember to do it regularly.

Choosing the right external drive size and connection type

External drives connect to your Mac through either USB-A, USB-C, or Thunderbolt. Check which ports your Mac has — newer Macs have only USB-C or Thunderbolt, while older models may have USB-A. You can use an adapter if needed, but a native connection is simpler and faster.

For storage size, measure your Mac's internal drive first. Open About This Mac from the Apple menu, click Storage, and note the total capacity. Your external drive should be at least that size. If you want Time Machine to keep multiple backup versions (so you can restore from different dates), choose a drive that is 1.5 to 2 times larger than your Mac's internal drive.

Common sizes are 1 TB (terabyte), 2 TB, 4 TB, and 8 TB. A 1 TB drive holds roughly the same amount as a typical Mac; a 2 TB drive lets Time Machine keep backups from several weeks or months back. Larger drives cost more but give you more backup history.

Formatting the external drive for Mac

When you first plug in a new external drive, your Mac may ask if you want to use it with Time Machine. Do not click yes yet — the drive needs to be formatted in a Mac-compatible format first. If your Mac does not prompt you, open Disk Utility (in Applications > Utilities).

In Disk Utility, select the external drive from the left sidebar. Click Erase at the top. A dialog will appear asking for a name (you can call it "Mac Backup" or anything you prefer) and a format. Choose Mac OS Extended (Journaled) for older Macs or APFS for newer ones running macOS 10.13 or later. APFS is faster and more efficient. Click Erase to confirm.

Erasing removes anything already on the drive, so if the drive is new or you do not need what is on it, this is safe. The process takes a few seconds to a few minutes depending on drive size. Once it finishes, the drive is ready for Time Machine.

Setting up Time Machine backups

Plug the formatted external drive into your Mac. Within a few seconds, a dialog should appear asking if you want to use the drive with Time Machine. Click Use as Backup Disk. If no dialog appears, open System Preferences (or System Settings on newer Macs), search for Time Machine, and click Add Backup Disk. Select your external drive from the list.

Time Machine will then ask if you want to encrypt the backup. Encryption protects your data if the drive is lost or stolen, but it also means you cannot access the backup on another computer without the password. For most people, encryption is worth the small performance cost. Click Encrypt Backup Disk if you choose this option.

The first backup will take several hours depending on how much data is on your Mac and the speed of the drive. Your Mac will continue to work normally during this time, though it may be slightly slower. Once the first backup finishes, Time Machine runs automatically in the background, creating new backups every hour as long as the drive is connected or nearby via Wi-Fi (if you have set up a wireless backup).

What happens after the first backup completes

After Time Machine finishes the initial backup, you do not need to do anything. The drive can stay plugged in, or you can unplug it and plug it back in later — Time Machine will resume where it left off. If the drive is unplugged for a while, Time Machine will catch up the next time you connect it.

You can check the backup status by clicking the Time Machine icon in the menu bar (top right of your screen). It shows when the last backup ran and how much space is being used. If you want to restore a file, click the Time Machine icon and select Enter Time Machine. You will see your files as they existed on different dates; select the version you want and click Restore.

Time Machine keeps backups until the external drive runs out of space, then deletes the oldest backups to make room for new ones. This means the longer you use the same external drive, the further back in time you can restore.

Manual backup if you prefer not to use Time Machine

If you do not want automatic hourly backups, you can copy files manually. Open Finder, navigate to the folders you want to back up (usually Documents, Pictures, Desktop, and Downloads), and drag them to the external drive. You can also create a folder on the external drive called "Mac Backup" and copy your important folders into it.

Manual backup takes longer than Time Machine because you have to remember to do it regularly, and you only have one copy of each file rather than multiple versions from different dates. However, it uses less space on the external drive and gives you complete control over what gets backed up.

If you choose manual backup, do this at least once a month, or more often if you create or change important files frequently. Write a reminder on your calendar or set a phone alarm so you do not forget.

Troubleshooting common backup problems

If Time Machine says the backup disk is full, the external drive is too small for the amount of data on your Mac. You can delete old backups to free space (Time Machine will prompt you to do this), or replace the external drive with a larger one. To replace it, unplug the full drive, plug in the new one, and repeat the formatting and setup steps above.

If Time Machine stops backing up and shows an error, try unplugging the external drive, waiting 10 seconds, and plugging it back in. If the error persists, open Disk Utility and run First Aid on the external drive to check for corruption. Select the drive, click First Aid, and let it finish. If First Aid finds problems it cannot fix, the drive may be failing and should be replaced.

If your Mac is very slow during backups, Time Machine is using a lot of processing power. You can pause backups temporarily by clicking the Time Machine icon and selecting Skip This Backup, or you can reduce how often backups run by opening Time Machine settings and adjusting the frequency (though this is not recommended for important data).

Frequently Asked Questions

Can I use the same external drive to back up multiple Macs?

Yes. Each Mac will create its own backup folder on the drive, and Time Machine keeps them separate. However, the drive must be large enough to hold backups from all Macs. If you have three Macs with 500 GB each, you need at least a 1.5 TB external drive to store all three backups with some room for multiple versions.

What if I unplug the external drive while Time Machine is backing up?

Time Machine will pause and resume the backup the next time you plug the drive back in. It does not delete or corrupt the backup if you unplug it mid-process. However, unplugging frequently can slow down backups over time, so it is better to let backups finish if possible.

Do I need to eject the external drive before unplugging it?

Yes. Right-click the drive icon on your desktop and select Eject, or drag it to the Trash. This tells your Mac to finish writing data to the drive safely. Unplugging without ejecting can corrupt the backup, especially if Time Machine is actively writing.

Can I back up to an external drive connected wirelessly?

Yes, if the drive is a wireless external drive or connected to a wireless router. However, wireless backups are much slower than USB or Thunderbolt. For the first backup, use a wired connection if possible, then you can switch to wireless for ongoing backups.

What should I do if the external drive fails?

If the drive stops working and you cannot access your backups, the drive itself has failed and cannot be repaired without professional data recovery, which is expensive. This is why keeping a second external backup drive in a different location is a good idea — if one fails, you still have the other.