
In the first case, the installer takes the current boot volume, by default named Macintosh HD, renames it Macintosh HD - Data, and removes most of the system folders and files.


There are two simple situations for the installer to cope with: upgrading from Mojave or earlier with a single boot volume, and a vanilla re-install. Then Catalina has to split earlier single boot volumes into its new Volume Group format, making a separate read-only System volume and leaving all the users’ files on the connected Data volume. First we have the continuing need to convert startup volumes from HFS+ to APFS, which has been mandatory since Mojave, even if your Mac normally boots from a hard disk or Fusion Drive. Recent versions of macOS have been exceptionally tough challenges for the system installer.
