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We have an iMac that is barely still compatible with El Capitan. Right now it runs Mountain Lion okay, and actually it has a smaller partition on the hard drive for another clean installation of Mountain Lion that we don't really use.

I have an El Capitan boot USB drive and an El Capitan install app, and I was thinking about using one of those to upgrade the clean install partition to El Capitan to see how it performs, but I'm hesitant and want to make sure things wouldn't be messed up.

So first of all, does this sound like a good idea? Second, would it be better to boot into the USB drive and choose to install to the clean install partition or should I put the install app on the clean install's Application's folder and run it from there?

Also would doing this risk causing the Mountain Lion restore partition to be deleted? Are there risks of things being messed up, not being able to make Mountain Lion the default boot partition, or other weird things El Capitan would do to the main partition?

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  • Can you edit your question and add the output from the commands diskutil list and diskutil cs list. I need to know if you are using core storage before trying to answer your question. These commands will not change your computer.
    – David Anderson
    May 12, 2016 at 23:29
  • @DavidAnderson I'll have to get back to you on that.
    – kal-al
    May 13, 2016 at 3:22
  • Mainly, I just wanted to know if you are using core storage. I was going to post an answer if you were not. I have both Yosemite, El Capitan and Windows 10 installed on my iMac.
    – David Anderson
    May 13, 2016 at 7:57
  • @DavidAnderson OK, see edit with a screenshot.
    – kal-al
    May 17, 2016 at 0:16

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Yes, you can go ahead and install El Capitan to the smaller partition.

You just need to run the "Install OS X El Capitan" application and select the proper destination volume. I would first use the Disk Utility application to erase this destination volume. You do not have to use a USB drive to install El Capitan.

During the installation, the Recovery volume following destination volume will be update with the proper code. The Recover volume for Mountain Lion will be unchanged. The result will be two different Recover volumes on your internal disk. Each will be given a different label so you can tell them apart.

During the installation, I was not asked, if I wanted to use Core Storage. In the event, you are asked, the answer should be No.

I verified this procedure by running the "Install OS X El Capitan" application from an internal Yosemite volume. I installed El Capitan to an external drive. The Yosemite volume was unchanged by the procedure.

I also have had Yosemite and El Capitan installed in different volumes on my internal drive for some time now. You can switch operating systems by selecting the Startup Disk in System Preferences.

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  • Thank you. To clarify, you recommend using Disk Utility to wipe the "clean install" partition as a new partition, running "Install OS X El Capitan" from within the main Mountain Lion OS, and then I will be able to choose the erased partition as the destination to install it? Also, why not use a USB disk or install it from within the test partition?
    – kal-al
    May 22, 2016 at 3:48
  • My answer is Yes to this: To clarify, you recommend using Disk Utility to wipe the "clean install" partition as a new partition, running "Install OS X El Capitan" from within the main Mountain Lion OS, and then I will be able to choose the erased partition as the destination to install it?
    – David Anderson
    May 22, 2016 at 4:04
  • I installed to a USB 2 external drive. USB 2 is rather slow. If you have USB 3, then things would run much faster. I am not sure what you mean by "or install it from within the test partition"
    – David Anderson
    May 22, 2016 at 4:08
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    You can copy the upgrade app to the secondary partition's application folder and run the upgrade while booted from the secondary partition.
    – David Anderson
    Mar 17, 2019 at 15:40
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    This is the first time someone waited almost 3 years before using one of my answers. I am glad the installation worked out.
    – David Anderson
    Mar 19, 2019 at 2:18

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