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I would like to install MacOS Catalina on an external drive (I have a 2 TB HDD), that when I turn on my Mac I can choose between Mojave, in the internal Drive, and Catalina, on the external Drive. Any Ideas?
iMac Line (2012 and Later)
Make sure to erase the external drive as GUID partition and APFS (top option) before installing Catalina to the external drive. Sometimes I find that when the installer reboots the computer sometimes it will boot into the internal drive instead of to the external drive to finish the installation. If this happens just hold down the Option key just after powering on the computer and select the orange icon for the external drive.
You can set either OS as the default OS to boot using the Startup Disk System Preference. To boot into the other OS just perform an Option Boot as I described earlier.
You can also install the rEFInd boot manager so that you don't have to remember to hold down the Option key to boot the one OS. rEFInd will present a menu on each boot so that you can select which OS to boot from.
Make sure to erase the external drive as GUID partition and APFS (top option) before installing Catalina to the external drive. Sometimes I find that when the installer reboots the computer sometimes it will boot into the internal drive instead of to the external drive to finish the installation. If this happens just hold down the Option key just after powering on the computer and select the orange icon for the external drive.
You can set either OS as the default OS to boot using the Startup Disk System Preference. To boot into the other OS just perform an Option Boot as I described earlier.
You can also install the rEFInd boot manager so that you don't have to remember to hold down the Option key to boot the one OS. rEFInd will present a menu on each boot so that you can select which OS to boot from.
I have a similar question.
I already have Mojave on an external drive.
I tried to set up a small Catalina installation on the same drive, copying user settings, but it failed to copy my user accessibility settings. System Preferences > Accessibility only contains a few of these. Configuring hidden accessibility settings can be a catch-22.
If I eventually have a larger drive, would an installation importing from my Time Machine backups import the needed accessibility settings?
I don’t recommend this. Some system behaviour, in particular iCloud, iCloud Keychain, Handoff, Airdrop, etc. operate on the basis of unique devices. If you have multiple boot sources, you no longer have unique devices. Of course, many things will work fine. But you can expect some odd behaviour as well.
Dual Boot Mojave/Catalina
