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I wish to keep my fully working MacBook Pro running macOS Mojave 10.14, but I also want to install macOS Catalina 10.15 beta on another drive, so I can choose which OS to use when booting.

I've installed the macOS Catalina beta profile from [Apple Developer Downloads].

Now the Software Update section of System Preferences lets me update to macOS Catalina 10.15. But before completing a 6 GB download, is this going to give me the option to install 10.15 beta on a separate partition or is this going to blindly update my existing 10.14 installation?

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    Also understand that you are running BETA software. So back up your stuff and don't be too surprised when something does not work as planned, and also this was released literally minutes before you posted this question. Any answer you get is an educated GUESS based on what macOS of the present and past has already done. So, again, back up your stuff and don't be surprised when stuff don't work and takes your work and/or files with it. – Steve Chambers Jun 3 at 20:07
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    I think the OP is aware of what beta software is, hence the question on dual boot. I always play with beta releases of macOS on separate volume and keep both isolated. – Tomek Cejner Jun 4 at 6:30
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Your existing installation won't be blindly updated. The macOS 10.15 Catalina beta installer will launch as soon as it is downloaded.

You can create a bootable USB installer by plugging in a USB flash drive and running the following in Terminal:

sudo /Applications/Install\ macOS\ 10.15\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/DRIVE

where DRIVE in the name of your external USB drive

You can choose to install macOS Catalina on an external drive, if you don't wish to interfere with your current installation.

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Good news. After the 10.15 beta download completes, the installer starts and after accepting the license, you are shown a screen where you can select where to install the OS. It defaults to the current disk containing 10.14 but there is a "Show Disks" button. Clicking it brings up a list of all other partitions attached to the computer. Many will be grayed out. The drive/partition you wish to choose must be formatted using APFS in order to be selectable as an option.

In my case I was able to install 10.15 beta to an external USB hard drive once I formatted it with APFS.

Naturally I made a full Time Machine backup and another full cloned backup to yet another external backup drive before running the beta OS. Be safe with beta OSes.

  • Good advice to not mix. Dual boot meaning external boot is good. Dual boot where people mess with APFS containers or try and hard partition yields a thousand questions on this site how to reclaim missing space after boot camp, manual partitioning, etc.. I love doing erase installs to see the new defaults. Beta OS or not. Then wipe and reinstall and migrate from a clone / other drive / time machine. – bmike Jun 5 at 16:56
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Yes, you can have a dual boot of your current macOS and macOS Catalina on a single Mac (without need for external SSD).

Pre-requirements:

  1. Access to macOS Catalina
  2. Compatible Mac computer
  3. APFS-formatted disk
  4. At least 20GB of free space (if you want to play with Xcode 11 beta, be prepared to have about 50GB to spare)

How-to:

  1. Open "Disk Utility" app
  2. Make sure your main volume is selected on the side panel
  3. Choose "Edit ➔ Add APFS Volume..." from app's menu
  4. "Add APFS Volume to container" popup should appear
  5. Name the new volume as you like (eg. "Catalina") and choose "APFS" format
  6. You can reserve the size for the new volume if you want ("Size options..." button), but you don't have to. By default, the new volume will share available space with your current volume.
  7. Download macOS Catalina installer (it should start automatically after downloaded)
  8. Choose newly created volume as an installation target
  9. Proceed with installation

Your Mac will restart several times and boot into fresh, new macOS Catalina installation. You will be able to choose the system on boot, just press and hold Option (⌥) key immediately after switching your Mac on. You can also change which system will boot automatically in "System Preferences ➔ Startup Disk".

Additionally, If you are using FileVault to encrypt your disk, macOS Catalina won't have access to your current macOS installation and all files you are storing with it until you provide the passphrase for FileVault. If you don't do this, you will keep your current files in isolation from the beta OS, which I recommend.

I am using the above setup to dual-boot macOS Mojave and Catalina without any issues on MacBook Pro 15", late 2016.

Please, remember to make a full backup of your system in advance - just in case.

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