I am still running macOS 10.11, now I wanna update to 10.12, but since some apps must be running on 10.11, so I need to keep this system and here is my plan:

  1. I use TimeMachine to backup my system 10.11
  2. Update the system to 10.12
  3. Restore the backup 10.11 from TimeMachine to an external HD, so I can boot it up there and I can use two versions of OS together.

Does this work? Will it cause any possible issues? License issue?

  • Frankly, I'd run El Cap as a VM [even more frankly, I'd make sure you have an actual clone of El Cap in case you change your mind, & if you do, don't wait too long.] – Tetsujin Mar 14 '17 at 20:36

Yes, this can work but not exactly in the way you've planned. Let me explain:

  • Yes, you can use TM to backup your computer, but you're not actually backing up a bootable copy of the OS.
  • Yes, you can update the OS on your internal drive to macOS Sierra.
  • Yes, you can restore the backup from TM to another external HD and use it as a second boot drive. But, before you can do this you will need to install El Capitan on that external hard drive and then boot your Mac from it and migrate your data etc from the TM backup to that drive.
  • No, there are no issues concerning software licenses in doing this.

Now, whether this is the best way for you to go about things is up to you. Another option, if you're internal drive has the capacity, is to have two partitions and have El Capitan on one and Sierra on the other.

Yet another option is to use something such as Carbon Copy Cloner or SuperDuper to make a bootable copy of your current system onto an external drive. Once you've tested it and are satisfied all is good, you can then upgrade the internal drive to macOS Sierra. This way you still get the two bootable drives.

If it was me I'd make sure I had a new external drive (in addition to the TM backup drive) and then install a fresh copy of El Capitan on that first. Once that's done, I'd boot up from that and migrate everything across from your TM backup. Then I'd do the upgrade to Sierra on your internal drive.

Once you've got everything working ok on both drives, you'll then want to think about how you manage some of your data. For example, you won't want to maintain two Photo Libraries, two iTunes libraries, etc. So you'll need to think about how you reorganise your files for use regardless of what system you've booted into, which ones will be duplicated or not, and also ensure you have a TM backup of all your data!

  • Note on installing El Capitan prior to restoring from Time Machine: If your Time Machine drive is connected when you turn your Mac on, and you're holding Option, there will be a Green Disk icon appear, which is a copy of Recovery HD on the Time Machine Drive, which you can use to Install El Capitan via the internet and should be able to restore your Time Machine Backup all in one step – FreelancerJ Mar 14 '17 at 11:24
  • Note on using Time Machine for multiple OSes on one Mac: Time Machine works out which Backup does with which Mac when multiple backups are happening to one disk using a "Machine ID", which is hardware specific. So it is best to use a second drive for Time Machine Backups for the Sierra side of things, and keep your existing backup linked to the 10.11 install. A second partition on the same drive also works. You will need to tell Sierra to REPLACE the existing backup drive when you select the new one, rather than use it AS WELL AS. Source: tried it earlier this year, got really messy :) – FreelancerJ Mar 14 '17 at 11:28
  • There is a support article on Apple's website about Time Machine needing to recreate a Backup from scratch after Main Logic Board replacement: support.apple.com/kb/PH25698 Also, for the first comment, it is mentioned at the end of support.apple.com/en-us/HT201250 – FreelancerJ Mar 14 '17 at 11:34
  • Also note that TM [in fact the OS itself] will not restore or migrate to an older OS. That was nearly my downfall recently - apple.stackexchange.com/questions/271104/revert-to-el-capitan – Tetsujin Mar 14 '17 at 20:30
  • Thanks @Monomeeth. It seems both ways are good to go, but installing the macOS first to the external drive can save the money to buy 3rd party software. This seems to be the best option! – RRN Mar 16 '17 at 15:47

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