Upgrading one partition to El Capitan while leaving the other on Mountain Lion 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?
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.