How to Back Up Your Mac Using an External Hard Drive

What backing up your Mac actually means

A backup is a copy of everything on your Mac — your files, photos, applications, settings — stored somewhere separate from your computer. If your Mac breaks, gets stolen, or stops working, you still have that copy. An external hard drive is a portable storage device you plug into your Mac with a USB or Thunderbolt cable. It holds that backup copy.

The reason to back up is straightforward: hard drives fail. They are mechanical devices with moving parts, and they wear out. Solid-state drives (the kind in newer Macs) can fail too, though less predictably. A backup means you are not starting from zero if that happens.

Key Takeaways

macOS includes a free backup tool called Time Machine that automatically copies your files to an external drive on a schedule you set.
You need an external hard drive with at least as much storage space as the data on your Mac — usually 500 GB to 2 TB depending on what you store.
The first backup takes hours because it copies everything; after that, Time Machine copies only new and changed files, which is much faster.
Plug in your external drive, open System Settings, find Time Machine, and select your drive — the setup takes about five minutes.
Keep your external drive unplugged most of the time and plug it in once a week or once a month; this protects against ransomware and power surges.

Choosing an external hard drive that will actually work

You need a drive with enough space to hold a copy of everything on your Mac. Check how much space your Mac is using: click the Apple menu in the top left, select "About This Mac", then click "Storage". That number is the minimum size you need. Most people should buy a drive that is 1 or 2 terabytes (TB) — that gives you room to back up multiple times as your files grow.

External drives come in two types: hard disk drives (HDD) and solid-state drives (SSD). Hard disk drives are cheaper and hold more data per dollar. Solid-state drives are faster and more durable. For backup purposes, either works. A 2 TB external hard drive costs between $50 and $100. A 2 TB external SSD costs between $150 and $250. If you are on a tight budget, an HDD is fine for backups.

Buy from a brand that has been around for years: Western Digital, Seagate, Samsung, or LaCie. Avoid no-name drives from marketplaces — when they fail, you have no recourse. Do not buy a drive that is already formatted for Windows; you need one that is either blank or formatted for Mac (APFS or Mac OS Extended).

Setting up Time Machine on your Mac

Time Machine is the backup tool built into macOS. It runs automatically once you point it at an external drive. Plug your external drive into your Mac using the USB or Thunderbolt cable that came with it. Your Mac will recognize it within a few seconds.

Open System Settings (the gear icon in your Applications folder or in the Dock). Click "General" in the sidebar, then scroll down and click "Time Machine". Click "Add Backup Disk". Select your external drive from the list and click "Use Disk". If your drive is blank or formatted for Windows, macOS will ask to erase it and format it for Mac — click "Erase". This erases anything on the drive, so make sure it is the right one.

Time Machine will ask if you want to back up files in the Trash and system files. For most people, the default settings are fine — just click "Set Up Automatic Backups". Time Machine will now back up your Mac automatically. The first backup runs when ready and can take several hours depending on how much data you have. Do not unplug the drive while it is backing up.

What happens during your first backup

The first backup copies everything on your Mac to the external drive. Depending on your Mac's speed and how much data you have, this can take 2 to 8 hours. Your Mac will slow down slightly during this time, but you can keep using it. Time Machine shows a progress bar in the menu bar (the clock icon at the top right of your screen).

Once the first backup finishes, Time Machine switches to incremental backups. This means it copies only the files you created or changed since the last backup. These backups run in the background and usually take 5 to 30 minutes, depending on how much you have changed. Time Machine backs up automatically every hour by default, but you can change this in Time Machine settings.

How to restore files from your backup

If you lose a file, plug in your external drive and click the Time Machine icon in the menu bar (top right of your screen). Select "Enter Time Machine". You will see your Mac's screen as it was at different points in time. Use the timeline on the right to go back to when the file existed. Click on the file and click "Restore". Time Machine copies it back to your Mac.

If your entire Mac stops working and you need to restore everything, you can do this during setup when you first turn on a new Mac or a freshly erased one. During the setup process, choose "Restore from Time Machine Backup" and select your external drive. Your new Mac will copy everything from the backup, including all your files, applications, and settings.

Keeping your backup safe from damage and theft

Do not leave your external drive plugged in all the time. Plug it in once a week or once a month, let Time Machine finish backing up, then unplug it and store it somewhere safe. This protects your backup in two ways: if your Mac gets infected with ransomware (malicious software that locks your files), the ransomware cannot reach the unplugged drive. If there is a power surge or lightning strike, the unplugged drive is not connected and cannot be damaged.

Store your external drive in a cool, dry place away from direct sunlight, heat, and moisture. Do not leave it in a hot car or near a heater. If you want protection against theft or fire, keep a second backup in a different location — at a friend's house, a family member's place, or a safe deposit box. You do not need to back up to the second drive every week; once a month is usually enough.

Check that your backups are actually working. Every few months, plug in your external drive and open Time Machine. Look at the dates and times of the backups listed. If they are recent, your backup is running. If the most recent backup is months old, something went wrong — unplug the drive, restart your Mac, and try again.

Troubleshooting common backup problems

If Time Machine says "Backup Failed", the most common cause is that your external drive ran out of space. Time Machine needs room to store multiple versions of your files over time. If your drive is full, delete old backups: open Time Machine, click the gear icon at the bottom, and select "Delete All Backups". Then plug the drive back in and let Time Machine start fresh. This erases all your old backups, so only do this if you have a second backup somewhere else.

If Time Machine keeps stopping or disconnecting, your external drive may be failing. Try plugging it into a different USB port on your Mac. If that does not work, the drive itself may be damaged. You will need to buy a new external drive and start backing up again.

If you see a message that your drive is not compatible, the drive may be formatted for Windows. Plug it in, open Disk Utility (search for it in Spotlight), select your drive, click "Erase", choose "APFS" as the format, and click "Erase" again. Then go back to Time Machine and select the drive.

Frequently Asked Questions

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

Yes. Each Mac creates its own backup folder on the drive, so they do not interfere with each other. However, the drive needs to be large enough to hold backups of all your Macs. If you have two Macs with 500 GB of data each, you need at least a 1 TB drive.

What is the difference between Time Machine and iCloud backup?

iCloud backup stores your files on Apple's servers over the internet. Time Machine stores them on a physical drive in your home. iCloud is convenient because it happens automatically, but you pay a monthly fee if you need more than 5 GB. Time Machine is free but requires you to plug in a drive regularly. Most people use both: iCloud for important documents and photos, Time Machine for a complete copy of everything.

How often should I back up my Mac?

Time Machine backs up automatically every hour by default. If you change files frequently, hourly backups are good. If you rarely change anything, you can change the schedule to daily or weekly in Time Machine settings. The important thing is that you back up at least once a week so you do not lose more than a week of work if something goes wrong.

Do I need to erase my external drive before using it for Time Machine?

If the drive is brand new and blank, no. If it has files on it, Time Machine will erase it automatically when you select it, but it will ask for permission first. Make sure you do not have anything on the drive you want to keep before you click "Erase".

What happens if my external drive fails?

You lose that backup, but you still have your Mac. If your Mac is still working, buy a new external drive and start backing up again. If your Mac also failed, you have lost everything. This is why keeping a second backup in a different location is worth doing.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.