Backup External Hard Drive to Another External Hard Drive on a Mac

By Simon Irons / August 27, 2018
backup external hard drive to another external hard drive mac

Can you backup an external hard drive to another hard drive on a Mac? The answer is definitely yes and I show you in pictures and in a video how.



Who is This Article Written For

​The ordinary user of a Mac who isn't particularly technical but knows they should have more than one copy of their files.

The Mac owner who has already helped themselves by having and taking regular Time Machine backups just in case of a drive failure or

has copies and hold specific types of files - say your photos on an external hard drive to save space on your Mac's internal drive.

What you want to do now is have a copy of the files on that external hard drive just in case essentially backup external hard drive to another external hard drive on a Mac.

If you don't know how to take a Time Machine backup to an external hard drive I tell you how to do that in my article on the site. How to backup to an external hard drive​.

Why Would You Do This?

​1. When you have all your files and photos or videos on your internal hard drive then that becomes a single point of failure.

Hard drives, even internal SSDs fail at some point and because of that failure you don't want to loose all the information stored on your Macbook or iMac.

So then you get yourself the best external hard drive you can find ​ so you can take another backup.

2. Your existing backup drive is producing errors and you want to save your backups to another hard drive.

3. Because you know if you loose your Macbook, say it gets stolen, you have a fire, or flood, or you spill your veggie vitamin juice all over your Mac. Then you'll need to have a backup you can access.

4. A quirk in the universal code of justice says that often when you need your backup disk that fails or gives you troubles as well. So a second backup is then needed.

The Best Principal

​The generally agreed best principle is to have a minimum of two backups, and to keep your secondary backup elsewhere. Perhaps take it to work and store it there or at a friends or relatives house or in a fire safe.

Because what if you make a silly mistake and ruin your backup or overwrite some crucial files? Or a file you are using becomes corrupt?

What Will You Get From This Article?

​By the end of this article you will know how to take a backup of one external hard drive connected to your Mac onto a second connected external hard drive.

Should a clone copy of the drive be what you need instead I have an article telling you how to do that here.

What Will You Use to Do the Backup?

What I advise and describe is how to use software you get for free on your Mac.

There are paid software solutions, which I talk about later.

The Mac software solution takes a bit of knowledge to use but is free - because it is right there on your Mac.

The paid options are slicker - that is what you pay for. You also need a version of the software compatible with the version of the operating system you are running.

So now we know the why and what, let me run through some possible scenarios for you

Scenario 1 - Backup Of External Hard Drive and Mac's Internal Drive

​You have your photos, videos or perhaps specific project files on an external hard drive to free up space on your Mac's internal drive.

You have or would like to have a second external drive to act as a Time Machine backup to your Mac's internal drive and to the external hard drive with your special files on.

backup external hard drive to another external hard drive mac

Needed Preparation

​1. The new external hard drive for Time Machine must be of sufficient size​;

  • To hold all the information stored on your internal hard drive,
  • Plus what is stored on the external drive with your special files on.
  • Plus sufficient free space for backup growth.

Let me net that out.

For Example, if you have 250GB used on your internal drive + 1TB of photos and other files on the external, you are going to need as a minimum a 2TB external drive to act as your Time Machine backup preferably 3TB for you to grow into.

2. The new external Time Machine drive must be formatted in preparation. The format needed is Mac OS Extended (Journaled) file system. If you are unsure of how to do this then read my document here​.

3. They both need to be plugged into your Mac at the same time so that your Mac can see both drives.

The Advantage of Using Time Machine

​1. Should you find a file has gotten corrupted you can always use Time Machine to go back to a clean version of the file. Time Machine will not overwrite files when it creates backups. It simply stores the new version and you can search back through time and find a non corrupted version.

2. It is inbuilt into the Mac's operating system so is there freely for you to use.

3. Time Machine takes a full backup the first time. Be prepared this backup can take some time. Following the initial backup Time Machine takes incremental backups. i.e only the changes made to your files up to that moment in time and is then a much faster backup

How to Backup an External Hard Drive on a Mac

​1. Have your Mac powered on and the two drives connected to your Mac. ​ You will see those drives on the desktop as Icons.

External Drive Icons On Desktop

External Drive Icons On Desktop

​2. Then you need to start up Time Machine preferences. To do this you click on spotlight search

Spotlight Search Symbol

Spotlight Search Symbol

​Type in System Preferences and on that window you will see the Time Machine application. Click on that.

Time Machine in System Preferences

Time Machine in System Preferences

​3. Once Time Machine is started, you will see the Time Machine setup window. There are a couple of things to check once the window is shown before you set everything up to backup your internal drive and your external hard drive to another external hard drive.

Time Machine SetUp Window

Time Machine SetUp Window

​Check that the tick box at the bottom is selected to Show Time Machine in the menu bar. This will be useful for later. And should the slider switch under the Time Machine logo and the words Time Machine be on. Click the slider to off

Time Machine Turn Slider To Off

Time Machine Turn Slider To Off

​This will prevent Time Machine kicking off a backup in the midst of you setting things up.

4. Now we are going to choose the external hard drive we want to hold our Time Machine backup.  In the example I gave you earlier this would be the 3TB drive - the drive big enough to take the backup we want and all the incremental backups (the changes) moving forward. You do this by clicking on Select Disk

Time Machine Select Disk

Time Machine Select Disk

​Then we select (click on) the name of the hard drive we're going to use.

Time Machine Choose Backup Drive

Time Machine Choose Backup Drive

​In this example here the drive happens to be called Backup Drive but you can name your Time Machine backup drive anything you want. You can have several backup drives set up for different things, just give them different names so that you can easily distinguish the one you're using.

Then click on the Use Disk button on the bottom right of the window.

5. Now we tell Time Machine what we would like to have backed up. By clicking on Options

Time Machine Options

Time Machine Options

​By default all external hard drives are excluded from the Time Machine backup list. All you have to do is include the external drive with your special files on.

On the Time Machine Options window you'll see a list.

Time Machine Excluded List

Time Machine Excluded List

​The hard drive we selected as the Time Machine backup drive in step 4 is on the list. It is excluded because it is the Time Machine backup drive. The external hard drive you also want to backup is here. And for this example this drive is called Media Drive and for you it would be your external hard drive with your special files on.

Click on the drive

Time Machine Click On Drive

Time Machine Click On Drive

​Then select minus to remove it from the exclusion list

Time Machine Click Minus

Time Machine Click Minus

and then click on Save.

That's it the set up is done. Just a couple more things to do to start off the backup.

6. Turn the main Time Machine slider from Off to On to start up Time Machine.

Switch On Time Machine

Switch On Time Machine

​As long as both external drives are plugged Time Machine will then take backups hourly for the first 24 hours, then daily for the next month, then weekly. But you can kick off the backup straight away by.

Heading to your menu at the top of your Desktop screen (remember earlier I told you to check the box to show Time Machine in the menu bar)

Click on the Time Machine Symbol in your top menu bar

Time Machine At Top of Desktop

Time Machine At Top of Desktop

​Then select Backup Up Now

Time Machine Backup Now

Time Machine Backup Now

​And Time Machine will kick off a backup for you. 

Time Machine may say preparing backup for a little while as this is the first backup of your internal and external drive but don't worry let it do its thing. The backup itself may also take quite a while, depending on the interfaces, and the type of hard drives you have. Again be patient and maybe leave it to do the backup overnight with your Mac connected up to the mains power.  You can check on progress by clicking on the Time Machine symbol in the top menu.

7. Once the backup is complete and should you want to plug out the hard drives connected, ensure you right click on the drive icons themselves on the desktop and select Eject to ensure your Mac writes all it needs to down to the hard drives and then they can be properly disconnected.

Right Click to Eject External Drive

Right Click to Eject External Drive

​Note that this Time Machine backup set up will remain and kick off whenever the external drives are connected or until you go in and change the Time Machine setup.

This set up will also backup any file changes made to your internal hard drive or your external drive with your special files on to the Time Machine backup drive.

Scenario 2 - Backup Of External Hard Drive Only

​Say you are happy with all the above except you do not want to backup your Mac's internal drive. You just want a Time Machine backup to another hard drive of your external hard drive with the special files on.

You will of course need a backup of your Mac's internal hard drive but you already have a specific external drive for that.

You will need to go into Time Machine and adjust the settings for the different types of backups you want to do.

How to backup a Hard Drive to Another Hard Drive on a Mac

​As before, the drives have to be formatted to Mac OS Extended (Journaled) file system. If you don't understand how to do this then read my article telling you how​.

Both external hard drives are plugged into your Mac at the same time.

Follow steps 1 to 5 as I've outlined above.

When you select minus to take the external hard drive you want backed up off the exclude list, you will need to add your Mac's internal drive to the exclude list as well. That way it won't be backed up.

The addition to step 5 would be as follows

5b. Click on the plus sign to add your internal drive (or indeed specific directories or files to the excluded from backup list).

Add To Exclude List on Time Machine

Add To Exclude List on Time Machine

​Either select your Mac's internal disk from the devices listed at the side of the window. It'll be the device without the eject arrow beside its name (you can't eject your Mac's internal drive because it is the drive you are using). Or

at the top of the window from the drop down list there.

Add Internal To Time Machine Exclude List

Add Internal To Time Machine Exclude List

​Then click on the Exclude button.

Continue to select plus and add what ever you wish to exclude (if you are only excluding specific directories and files).

Once you are done click save and your Time Machine backup settings are saved.

Save Excluded List Canges to Time Machine

Save Excluded List Canges to Time Machine

​Then continue with steps 6 and 7 above.

You can always change the Time Machine settings once the backup is completed, back to use a different backup disk drive when you're done.

Now you know how the principal works and how to do it. 

YouTube Video on Backing up An External Hard Drive

To Another External Hard Drive on a Mac.

For the more visually orientated out there who also like to see a video on the subject - Thanks to Daniel Sato for this video, he shows you how it is done on a Mac.

Using Time Machine to Backup an External Hard Drive

Video Credit: Daniel Sato

Other Third Party Options

There are paid software solutions that will backup and copy one external hard drive onto another one.

Advantages

  • ​They are specific to this task so you have more options available.
  • They will allow multiple backup scenarios/scripts to be created, where as the Time Machine set up needs changing between backup scenarios
  • Will allow for network back options

Disadvantages

  • ​You have to pay for them, though you do get a free trial period with some restrictions in use in some cases
  • You may find them complex to download and set up particularly if you are not very technical
  • Will need to check and download the compatible version for your operating system, particularly before upgrading your Mac's operating system

The ones most often used on a Mac are;

Last words

I hope you found my article 'Backup External Hard Drive to Another External Hard Drive on a Mac' useful in showing you how a standard piece of software freely available on your Mac does allow you to backup not just your internal drive but an external one attached to your Mac as well.

Should the worst happen and you loose your external hard drive with your photos, videos or project files on you can use your Time Machine backup to restore.

Indeed you can use your Time Machine backup of your Mac's internal hard drive to boot from should your Mac's internal drive fail. You can boot from your Time Machine backup.

When you need an exact copy of an external hard drive on your Mac a clone copy of your external drive to another external hard drive may be just what you need.

Although the best way of creating a bootable drive is to create a clone copy of your Mac's internal drive - its boot drive, and this article​ will tell you how.

​And if you want to learn how to restore from a Time Machine backup I have an article on that too.

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