After a time machine migration from an old iMac (non-fusion drive) to my new iMac (Fusion Drive), I noticed that in Disk Utility, my Fusion drive lists two volumes: "Macintosh HD" and "Macintosh HD – Data". In Finder, I just see "Macintosh HD". Is my drive split or is this normal behavior in Disk Utility?
Best Answer
This is normal for all Catalina system/boot volume layouts independent of the number of physical disks involved.
The various main partitions (here disk0s2 & disk1s2) of the single physical disks of a Fusion Drive are combined as physical stores to one container: disk2. The predecessor (CoreStorage) did the same.
The whole container (here disk2) is divided into various APFS volumes with different roles: Preboot, Recovery, VM, Data and System. Each volume can be enlarged or shrunk dynamically.
The last two are combined in one APFS volume group.
You get all available APFS volume groups by entering
diskutil ap listVolumeGroups.The system volume (role: System, name: Macintosh HD, device: disk2s5) is mounted read-only. The necessary read-write data volume (role: Data, name: Macintosh HD – Data, device: disk2s1) is mounted to /System/Volumes/Data.
By default various folders of the Data volume are firmlinked to various folders of the System volume. They are needed to add user installed apps/binaries/frameworks etc. and user content (which requires a r/w volume) in a transparent way. The list of a all current firmlinks is available at /usr/share/firmlinks.
The two devices in the APFS volume group (disk2s1 & disk2s5) mustn't be confused with the two physical devices (disk0 & disk1). Both APFS volumes may partially reside on both physical disks.
At least the APFS volumes with the roles Data and System are shown as separate devices in Disk Utiliy. I expect the volumes with the roles USER, Undefined and Installer (which don't exist in the standard partition scheme) to be also shown in Disk Utility – I haven't tested this though.
Further reading and pics of the various stages of Catalina APFS volume schemes: