Installing MacOS software on external disk

The Catalina installer will format the target drive using APFS, automatically. If the target drive is formatted HFS+, then the installer will convert the drive to APFS without existing data loss.

This information has been circulating since before Catalina's name became public. APFS, only, and 64-bit software, only. Catalina will do just fine with drives formatted HFS+, just not as a boot volume.

See Disk Utility User Guide.

See…

Is the public beta software confidential?

Yes, the public beta software is Apple confidential information. Don’t install the public beta software on any systems you don’t directly control or that you share with others. Don’t blog, post screen shots, tweet, or publicly post information about the public beta software, and don’t discuss the public beta software with or demonstrate it to others who are not in the Apple Beta Software Program. If Apple has publicly disclosed technical information about the public beta software, it is no longer considered confidential.

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mackiedoo wrote:

Starting with MacOS Catalina does MacOS have to be installed to APFS formatted disks?

No.

There is no higher level OS than Catalina for the Mac at this time. Catalina will be superseded in the Fall by Big Sur which is in Beta testing. We cannot discuss Big Sur on these forums.