With the public beta out for MacOS 10.15 Catalina, you can be a thriller and will test it in progress version. But maybe you want to secure your bets. Previously, you had to partition the boot disk, which can do a lot of work, or get an external drive, preferably SSD, and install and boot from it.
However, there is a better way to have your Catalina and start it as well. There is even a path of Apple documents and recommend.
With Apple's non-brand new APFS file system replacing the long-lasting HFS +, drives are no longer organized into partitions, but volumes and containers. A container receives a pool with a fixed storage space on a station when it is configured, but containers may have several amounts. Volume divides all free space in the container without requiring any other alignment ̵
(Think of this as having a measuring cup with a quarter of water in it and a set of a quarter jars. Can divide the measuring cup's water in some way among the cups, including putting all the water into just one cup or pouring it from any cup back in the measuring box.)
How this works to your advantage with Catalina, that if you have enough spare parts in the main container to handle Catalina – a few tens of gigabytes, but more preferably – you use Disk Utility to add to a value in your main container, and then install Catalina in that volume. You can then use the Startup Disk preference pane to switch between your volumes without involving an external hard drive at all.
This can still be useful after Catalina is released if you want to keep a Mojave volume active for 32-bit apps that no longer run in Catalina. I will describe a strategy below for that.
Add a Catalina volume to your main container
Warning! Whenever you make changes via Disk Utility, bad things can happen unintentionally. Make sure you have an updated clone or back up your drive before continuing.
Start disk utility.
Make sure View> Show All Devices is selected.
Select your starter drive main container in the driving list on the left. It can be called something like "Container disk2."
Click the + (plus) sign above the volume button at the top of the screen.
Name the volume somewhat descriptively, leave other options, and click Add.
Now you can review our steps to install Catalina's public beta. Absolutely critical, you are sure to choose the new volume that has just been created. When I tried this for the first time, this volume didn't show up immediately, I needed to click View all disks, but also try the installation sometimes before I could choose my new volume. It is unclear why, but it finally worked.
After Catalina has installed and you have set it up with a new account, you should return to your previous MacOS installation as you wish unenroll ] that system from the public beta so that it does not attempt to install Cat alina updates!
Open the boot disk startup pane.
Choose your previous macOS boot volume. (You may need to click the lock at the bottom left and enter your password.)
In your older system, open the Software Update pane.
Below the left-hand note that Mac is registered in public beta, click the Details button.
Click Restore Default Values and follow messages.
Keep Mojave active after Catalinas released
Usually when a new OS comes out, you run the update process and it upgrades system files in place. When it restarts, the older version of macOS has been wiped away.
However, with the container and volume access, you can migrate inside the container and have the advantage of retaining the old Mojave backward compatibility installation with older apps. (Someone who used to rely on virtual hardware to run old versions of macOS, and it's still a viable option, especially if you see yourself needing to run the older version for years.)
This approach benefits of how the container shares free space among its volumes. When Catalina is released, update the Catalina volume, and then transfer Mojave-associated programs, files, and settings from the Mojave volume to the Catalina.
Then you can delete all the Mojave apps, files, libraries (like pictures, iTunes and iMovie), and other settings you no longer need there. This space is then released into the container and available for the Catalina installation. But you can still boot into the Mojave as needed and use programs that are not compatible with Catalina.
If you have enough storage space to duplicate everything through migration, you can simply start with Catalina, run Migration Assistant, and use the Mojave volume as the Origin. Delete unnecessary files in Mojave when done.
If you do not have enough storage space to duplicate all your Mojave files and the like, first make a Time Machine backup or clone the Mojave installation to a remote drive, then delete the files you no longer need in Mojave. to free up space for the transfer to Catalina, and finally use Migration Assistant in Catalina to withdraw from Mojave's remote station.
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