I'm rarely an early adopter of new software, but I'm taking the plunge today and buying Mountain Lion. I don't know if it will break anything important (I run a few rather obscure programs on my mid-2011 Macbook Air), nor if I will like it. I have a current Time Machine backup, but no bootable clone. I can afford to be without the use of my computer for a few hours should things go horribly wrong and I need to restore it.

I'm considering charging ahead and installing Mountain Lion with just a Time Machine backup (in part so I can get up and answering ML questions on this site!). Is this a reasonable course of action or incredibly stupid, and if so, what should I be doing instead?

  • Personally, I would never feel safe with my data on just one backup disk. I may be super paranoid, but I keep three backups of my personal files, in two different locations – and two of the three sets are each on a mirrored pair of disks, for a total of five backup copies. I would suggest having an extra copy of your home directory, at least, in addition to the TM backup. The chances of your backup disk being unreadable when you need it may be slim, but not zero. So how much do you value your data? Only you can decide. Jul 25, 2012 at 14:39
  • Well, everything important is already on DropBox to begin with, and this machine doesn't store my photos or anything truly irreplaceable.
    – Daniel
    Jul 25, 2012 at 14:40
  • Ah, you're probably good to go, then. But I'll leave my comment for the benefit of other visitors. Jul 25, 2012 at 14:41
  • FWIW, I have three separate backups of my photo library. :-)
    – Daniel
    Jul 25, 2012 at 14:42

3 Answers 3


Here are the things you'll want in case things don't go well with the install over your internal drive with your apps, settings and files:

  1. An external USB drive with a Recovery HD (4 GB is plenty for this)
  2. Time (Downloads could be slow and error prone today)

Dive in is my advice given your recent hardware, speed of the machine and storage and the framing of your question (you've asked the right details to be aware of the potential downsides).

Do try booting to the recovery USB drive once and see that it shows your Time Machine drive as a valid place to restore Lion. I have had people find one USB port was dead when they tried to use both - I know this is paranoid, but update time is when most people find out about issues (software corruption and hardware problems) since it changes things at a very low level. Machines can be infirm enough to keep running and fail when faced with a re-install or migration of data that a major OS upgrade entails.

If you were paranoid, you could also add another external drive with 20GB free to your list of things needed. Just installing a clean version of Mountain Lion is always safer than installing onto a system with data and settings. By migrating the contents over to be sure all your third party apps are ready for the new OS. This is a nice alternative to making a bootable backup.

Re-doing the installer a few days later on your internal drive is easy work (or using Disk Utility to erase the internal drive and "restore" the external image to the internal drive doesn't take that long for Air sized images.

Use Disk Utility to move external data to an internal drive


I agree with all recommendations. I was one of those unlucky enough for Mountain Lion not to like my internal hard drive, resulting on a panic kernel error upon trying to reboot. Suppossedly because it had problems with permissions requiring repairs.

Fortunately I had an external thunderbolt hard drive to reboot from. None of the supposed permissions problems were confirmed with Disk Utility.

Nevertheless I decided to reformat the internal hard drive and perform a clean install of Mac OS X Lion.

I will then restore the Time Machine backup I had just made before attempting the upgrade to a Mountain Lion.

I guess the time is not yet ripe enough for me to risk an upgrade...

  • UPDATE: rebooted with Cmd-r. Reinstalled Lion 10.7.4 ove wifi. Restored my Time Machine backup. Everything is now in working order. By the time I decide to upgrade to Mountain Lion, the kind of errors I got should be a thing of the past. And of course, all of the applications I use daily should be fully 64-bit compliant. Unfortunately, that is not yet the case for me... Jul 31, 2012 at 9:38

Your solution seems good enough Time Machine only, however I would recommend an additional bootable clone of your primary OS drive would be more ideal, allowing you to just boot up from or clone the non upgraded copy to the primary drive if needed.

  1. The advantage is you would be exactly where you left off at before upgrading to 10.8, as opposed to a potentially different restored time machine state, that may or may not be missing some minor settings.

  2. This should save you in the worst case situation, since you would not have to re-install 10.7 and then try to restore from Time Machine backup.

  3. Its an extra backup up copy, the more copies the better...

  • 1
    In the older days when drives were smaller, I loved making bootable backups. Now that drives are bigger, I'm much more a fan of installing cleanly onto an external partition (resizing it up/down as needed) and then running a test migration. Then I can clone that back or re-do the install on my main drive. Your advice is good - I just wanted to compare the two approaches. Rather than taking time to maintain a bootable backup - I keep my bootable backup on my internal drive until I know the OS is a good fit and do my testing on an external drive.
    – bmike
    Jul 25, 2012 at 15:14

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .