I have a 5TB external disk (WD My Book), which I use as my main hard drive. It is always connected via USB to my Mac. I've just bought another 5TB external disk (Seagate backup plus portable), where I always would like to have a 1-1 copy of the whole date from the first disk.

I am looking for an application or a simple way, which does this:

  • Everytime I stick the 2. usb drive to my Mac, it should cleverly checks the files which are removed, or added to the first drive, and makes the same changes on the second one. It can of course remove all files, and copy all files from the first drive, but since I have multiple TB data, it will not be efficient. And most of the time, I will have not so many changes anyway.

Thanks in advance!

  • Generally speaking, I would use rsync, which is already included in macOS, and an Automator Folder Action that watches /Volumes and when you plug in the second drive it checks to see if the first drive is also connected, and if it is, it then syncs the drives. Dec 19, 2019 at 15:10
  • Would you please clarify what you mean by "which I use as my main hard drive"? Are you booting from the WD My Book, so in System Preferences > Startup Disk it's selected or is the WD My Book just just being used to store user data and such? Dec 20, 2019 at 15:08

3 Answers 3


I've just bought another 5TB external disk (Seagate backup plus portable), where I always would like to have a 1-1 copy of the whole date from the first disk.

It’s not a sound strategy to back up a backup.

I write about this in much more detail in another related answer, but to summarize here, you should have distinct backup jobs, not copies of existing backups.

Having an “identical copy” of a drive is good for redundancy and the correct tool for that job would be a product that implements RAID where the drives are mirrored. Manually (meaning non-RAID) syncing drives falls short of being a good backup solution because if there are issues with the original backup (i.e. file corruption), the same issue is replicated to the other drive making the backup useless.

Syncing is a tool best used when you need to have files accessible in/from different locations and are typically not being simultaneously accessed (changed).

What you should do with your current setup is continue to use the MyBook for your daily backup and then create a new backup job that does a differential (only what has changed) backup when you plug in the Seagate.


You could do it yourself with rsync but if you want a better, more Mac-like / GUI experience, I’d recommend Carbon Copy Cloner.

It will only copy over the files which have changed, and it can be set to sync whenever a disk appears, or it can manage mounting and unmounting the second drive on a schedule. I’d have it set to run at like 1:00 a.m. so as to not use system resources while I am using the computer.

I also own ChronoSync and SuperDuper, but I think CCC is the best / easiest tool for the job. I’m pretty sure there’s a free trial period, so you can test it out and see how it does.


You are looking for a file syncing app. There are many to choose from.

Chronosync and SuperDuper, are apps that I have used for different purposes, but there are many others.

PS: Answer me this: if you accidentally delete a file on your 1st external, and then the 2nd drive synchronizes and deletes the file: how will you recover that file?

  • i do not accidently delete anything. I did not have to write “backup” actually. you are rigth.. i need a sync. thnks. but do they have this functionality like as soons as the 2. disk plugged in, starts to sync or sth
    – akcasoy
    Dec 19, 2019 at 12:49
  • All of them have a range of methods for triggering syncing. Resilio is the best at real-time live syncing, IMO. Allan's answer makes my point about deleted files more clearly.
    – benwiggy
    Dec 19, 2019 at 14:31
  • 1
    As far as I know, you cannot use Resilio Sync to sync two folders on the same Mac. The app will recognize that the folder already exists and will refuse to add the 2nd instance.
    – TJ Luoma
    Dec 19, 2019 at 15:25
  • Ah! Good point. I use it to sync two Macs, and wasn't thinking straight!
    – benwiggy
    Dec 19, 2019 at 16:40
  • ChronoSync can be configured to trigger the sync immediately after both drives have mounted. It also has the ability to make a mirror sync, and to only sync the files that have changed. In my opinion, out of all the apps that have been suggested so far, it is the best one to do what you wish.
    – user128998
    Dec 19, 2019 at 19:34

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