I have got an external hard drive (A) that contains my iTunes & Aperture libraries. It's not always plugged to my MacBook.

What's the best way to make a backup of it on another external hard drive (B) ?

Wanted features :

  • fully automated (that executes itself hourly, or something like that)
  • incremental (in order to be fast)
  • only activates when both disks (A & B) are plugged in
  • works with multiple backup disks (in order to have a backup at home and another at work)
  • discrete indication that a backup is in progress
  • ability to pause or stop backup (useful if I am in a hurry and have to disconnect external drives)

It would be nice if backup disks could be networked, but that's not mandatory.

I would have loved to use Time Machine in order to have an history, but it doesn't seem to work well with external hard drives that aren't always connected and I don't trust it for that use case (my internal SSD is backed up thanks to Time Machine, though).

Online backups aren't adequate because I have a very poor internet connection.

  • I would compromise on your criteria and use Time Machine to back both drives to a third. With multiple backup destinations on Mountain Lion it is even easier to control where your data is going. The command line tools let you script things as much as you care so you might not even have to compromise if you don't mind programming TM.
    – bmike
    Commented Sep 2, 2012 at 11:09

5 Answers 5


Another possible option: use rsync and a launchd job to execute it when the disk is mounted. I believe Time Machine uses a similar approach to running on disk mount. For the hourly execution, you can use cron. The discreet notification can be done in a number of ways (for instance sending a mail or the Growl extension which allows notifications to be sent from the command line). killall rsync should stop the backup safely if you need to pack up and go. Also rsync is also entirely capable of networked syncing.

It will take a bit of work to set up, but the advantages are:

  • it's versatile (see man rsync, they pretty much thought of everything)
  • it's discreet
  • it doesn't cost anything
  • rsync is quick

You may find this helpful for launchd jobs involving disk mounting.

Edit: I should add that the --partial option to rsync will mean that the backup is resumed after killall rsync.

Edit 2: You can use a Control Plane context instead of launchd if you prefer.

  • This solution seems interesting, given I don't find any commercial software that suits me. I'm digging this one (with a bit of UI, maybe a widget or a menu item) while I'll continue to look for an existing solution (it is (or should be) easier to setup and maintain).
    – olivier
    Commented Sep 11, 2012 at 7:01
  • It would be helpful to include some rsync examples that users can run rather than telling them to "RTFM" (i.e., "see man rsync") as command line can be a bit daunting, especially when it comes to shifting around our precious files.
    – user10355
    Commented Dec 22, 2013 at 1:01

I'm continually amazed by Carbon Copy Cloner's speed and advanced features. I just double-checked and confirmed that it can be used to clone external drive to another external drive.

It's free for a month's trial period with full functionality. I strongly recommend this app as the best option I'm aware of.

Alternatively, there is another excellent program Clonezilla (I've actually linked you to the company's Tuxboot page on Sourceforge, because Tuxboot is an amazing program for booting from USB flash drives):

Tuxboot helps you to create a bootable Live USB drive for Clonezilla live, DRBL live, Gparted live and tux2live. It is modified from unetbootin and runs on both MS Windows and GNU/Linux. You can choose to download the latest version of ISO file then create the live usb.

nb. You can actually use unetbootin on a Mac - there is a dmg download on Sourceforge I can't link to as new users are limited to two hyperlinks. I think to use Tuxboot on a Mac, you'll need to use CrossOver or Wine or one of those Windows emulation programs (I have done this successfully with CrossOver when I couldn't get the tar.gz installed).

But really, Carbon Copy Cloner has all the features you list and more so that's got to be your best option, I believe.

  • Clonezilla page says "Differential/incremental backup is not implemented yet." so does not meet the OPs requirements
    – mmmmmm
    Commented Aug 19, 2012 at 11:27
  • 1
    I wasn't aware of the (quite complete) scheduling features of CCC. I was hoping for a more streamlined experience (I find the UI a bit clumsy), but CCC should do the trick.
    – olivier
    Commented Aug 19, 2012 at 12:54

I have used Time Machine for years to back up external hard drives which were not connected 100% of the time. Occasionally, it will require a full rescan of the drive to make its backup, which will obviously take longer than usual, but the system has no way of knowing that the drive hasn't changed since the last time the system saw it.

Why don't you trust it?

  • I don't trust it for two reasons : 1/ In a test I made, my external disk was excluded right after I told Time Machine not to exclude it (on Lion). I did not dig further because of 2/ "However, it's best to leave such a drive connected as much as possible, so you don't run afoul of Time Machine's "thinning" schedule; if you connect an external HD and run a backup, then disconnect it, the backups could be deleted in as little as 24 hours." (pondini.org/TM/32.html)
    – olivier
    Commented Sep 2, 2012 at 10:26
  • If you back up to two external drives, or two volumes on a single external drive, one each for the internal and external data drives that you have, you shouldn't have any thinning issues. But I think that quote is correct only in terms of versions – your most recent backup will always be present and won't go away due to how TM works. Btw, I hope you're backing up somehow now, because TM is still way better than nothing. But again, in my case, I saved my iTunes library thanks to it, so I'd suggest using it for now.
    – lensovet
    Commented Sep 2, 2012 at 10:59
  • In my understanding, my most recent backup does not especially include a backup from my external disk, that's the problem. As a temporary solution, I'm doing manual clones so my medias are backed up.
    – olivier
    Commented Sep 11, 2012 at 6:43
  • Where is this understanding coming from? If you go into Backups.backupdb on the TM volume, and look inside the "Latest" backup, is there a folder corresponding to your external drive? If so, your understanding is at odds with reality. If not, reconnect the external drive and verify that it's not in the exclusion list in TM preferences.
    – lensovet
    Commented Sep 12, 2012 at 5:34
  • 1
    Sorry, wanted to insert a newline. For how Time Machine works, see this great writeup: arstechnica.com/apple/2007/10/mac-os-x-10-5/14
    – lensovet
    Commented Sep 12, 2012 at 19:37

My internet connection is pretty poor as well which is why I have turned to CrashPlan to perform local backups of my MacBook Air and always connected external HDD to a sometimes connected storage device. Indeed, CrashPlan also allows you to back up to your drive at work over the internet for free as well - this point however is moot since your internet connection is poor.

  • I don't want to rely on the free version of CrashPlan, because they could make the subscription mandatory anytime. And I don't want to pay a subscription for a local backup software.
    – olivier
    Commented Sep 11, 2012 at 6:51
  • At some time in the future conditions impacting my workflow may (insert will if you prefer) change, at which point I will revaluate and alter my workflow. That is the nature of tech. I'm a SuperDuper guy myself (I like clones for local backup) but I do use Crashplan Central for offsite backup and wouldn't hesitate to use the software for a local (or self-managed offsite) backup. The odds that the software just stops working anytime soon are pretty low and the benefits are high.
    – jaberg
    Commented Dec 21, 2013 at 18:16

I tried using External Hard Drives but i'm not satisfied coz it can be stolen or lost so I decided to back up my files using Cloud. Now i'm happy with it.

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    Commented Nov 14, 2014 at 11:45

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