APFS added second 'Data' Volume?

[Doc69]

Before APFS I used to partition one external SSD into several different partitions so that I could install (and boot into) different versions of MacOS for testing. No problems here. Now I tried to do the same thing with APFS but something weird happened. When I was about to partition the SSD, I was given the choice to instead of partitioning, just add new volumes. I guess this shares the space without pre-allocating it, which seems great. I first installed Catalina on a clean external SSD. I then added a second 'Mojave' volume and installed Mojave on that. However, after doing so, in addition to the Catalina volume, I now had two 'Mojave' volumes, one that is called 'Mojave' and another one called 'Mojave - Data'. Both these volumes have the same content, i.e. a full install of Mojave and shows up in the APFS container in Disk Utility. I then added a third volume, 'Mojave Test', and now the Catalina volume is the one that has an additional 'Data' volume instead of the first Mojave volume.

Does anyone know what's going on here? Thanks!

 

[Fishrrman]

That has nothing to do with "APFS".
It's Catalina that has created that "second" data volume.

 

[madmin]

@Doc69

Here's an depth article from Howard Oakes published today, that explains all the ins and outs of the new system volumes in Catalina:

Catalina Boot Volumes

macOS Catalina is the first version of macOS which boots not from a single volume, but from two: a read-only System volume named by default Macintosh HD, and a writeable Data volume with the defaul…

eclecticlight.co

Other articles are available for Mojave etc. You should be able find all the answers you need there.

You may also find Rich Trouton's instructions for creating 'firmlinks' on Catalina useful:

Creating root-level directories and symbolic links on macOS Catalina

One of the changes which came with macOS Catalina was the introduction of a read-only root volume for the OS. For users or environments which were used to using adding directories to the root level…

derflounder.wordpress.com

 

[Doc69]

OK, thanks guys! After reading the articles and doing some more research, it looks like it happened because I installed Catalina on an external drive when I was booted up in Mojave. That way the Catalina 'Data' volume didn't not link properly with the 'System' volume, so both shows up on the desktop when I'm booted into any of my Mojave volumes. But when actually booted into Catalina, the 'Data' volume is not visible on the desktop, only in Disk Utility.

Here are another couple of good articles if anyone is interested in learning more:

Why you might see a new “Data” disk in Catalina

MacOS now splits the operating system and data files into two volumes, but they should never appear on the desktop.

www.macworld.com

Working with APFS Volume Groups - Bombich Software

bombich.com