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Question: Multiple external hard drives with time machine

Good day all,

I am looking for advice on the best way to set up my externals to work with time machine.

I have 3 externals. Drive a, b, and c.

Drive a holds my work.

Drive b is my work backup

Drive c is my computer and work backup.

I want drive b to backup drive a, and drive c to backup my computer and drive a ( or b, as they both will have the same information stored).

How would I set up these drives and time machine to make this happen?



Posted on


Dec 16, 2017 8:18 PM in response to Hebbie70 In response to Hebbie70

  1. First, you need to tell us how large each drive is that wish to backup.
  2. Second, how large are each of your backup drives.
  3. Third, visit Pondini's Time Machine FAQ for help with Time Machine. This is an extensive site with most everything you may need for using Time Machine. You will not find a better resource for Time Machine in one location. Due to the passing of James Pondini, the site is no longer being updated. Therefore, some of the information may be a little outdated. Please visit The XLab FAQs and read the FAQ on backup and restore. Also, read How to Back Up and Restore Your Files.

Dec 16, 2017 8:18 PM

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Dec 17, 2017 7:25 AM in response to Kappy In response to Kappy

Hi Kappy,

My a and b drive are 4 tb.

My c drive is 8 tb.

I have plenty of room for everything.

My question is more around how I backup external a onto external b using time machine.

I have done it with my last computer, but my 11 year old Mac died and I replaced it.

Hence I need a refresher in time machine as I am resetting everything back up.


Dec 17, 2017 7:25 AM

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Dec 17, 2017 10:20 AM in response to Hebbie70 In response to Hebbie70

You have to have another Time Machine configuration in which the external backup drive is selected as the Time Machine backup drive and the other external is the source drive. It's my understanding that Time Machine can now have multiple configurations in the same setup, but I don't use Time Machine so I've never checked that out.

I use Carbon Copy Cloner for my backups. It allows a simple setup for each desired backup. Each setup can be scheduled for different times of day, frequency, and day of the week. Unlike you, I have one backup drive for each drive I backup. Putting more than one backup on the same drive puts both backups at risk should the drive become corrupted or fail. Each backup occurs at half-hour intervals on a backup day. For my system, 30 minutes is ample for each backup. Like you, I do keep multiple backups in case a drive fails or becomes corrupted. You can never have too many backups.

I'm not sure if there is much more I can tell you that you don't already know. I am not sure that Time Machine can now handle multiple backup configurations, but I seem to remember that being part of Time Machine's major rewrite several years back.

In the Time Machine preferences, you select your first backup drive. Configure your first backup. Now, from the list of disks available choose the next backup disk. You are prompted if you wish to change the backup disk or to also use the newly chosen disk. You apparently repeat this until you have selected each desired backup disk. Then make the second part of each Target - the Source disks. Remember that Time Machine sets a Source disk by excluding the disks you don't want for this backup configuration.

Maybe this will help get you started.

Dec 17, 2017 10:20 AM

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Dec 17, 2017 10:55 AM in response to Hebbie70 In response to Hebbie70

I use both CCC and TimeMachine. TimeMachine is for recovering a deleted file or the ability to get an earlier version of a file. I use ccc to have a bootable copy of my startup drive. With timemachine, should you have a drive failure, you will have to restore to a new drive. With ccc you have a clone of the drive. You could go with a raid setup to protect against harddrive failure. You should have your most critical stuff off site.


Dec 17, 2017 10:55 AM

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Question: Multiple external hard drives with time machine