We may have been using APFS alongside HFS+ for four years now, since the former was introduced in High Sierra, but there’s still plenty of confusion as to how they work alongside one another. This primer explains how they differ structurally, and how basic tasks such as adding a new volume differ.
Unlike some file systems, unless you operate disks together in a RAID array, the two native Mac file systems don’t normally span across physically separate disks. Each disk is formatted using a GUID Partition Table (GPT), which maps out separate storage areas in partitions. Being separate, they can’t share storage space, and are normally considered to have fixed size, although you can repartition a disk, which can usually be accomplished without losing any of the existing data.
Partitions work differently depending on whether they are HFS+ or APFS.
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