Fusion Drive: Why One Disk Alone Shows Half the Data
Fusion Drive is not a cache. It is a single logical volume whose blocks are distributed across an SSD and an HDD, with metadata on both.
Structure
Core Storage (or APFS on newer systems) combines the two devices into one logical volume. Hot data lands on the SSD, cold data on the HDD, and the mapping is dynamic. The SSD typically holds more of the metadata.
Recovery
Image both members, parse the volume-group metadata, then reconstruct virtually. If the SSD — which carries most metadata — is dead, recovery drops to file-signature salvage.
FAQ
Can I mount the HDD alone?
You will see a partial filesystem at best, and macOS may offer to initialise it — which destroys the metadata.
Is the SSD replaceable?
On older Intel Macs yes, but pairing with the existing volume group is not something the OS will do for you.