I have read that APFS was optimized for SSDs, but that it's not an optimal choice[ for HDDs. However, I just got my iMac (Model 17,1; OS X Mojave, 10.14) back from repair, and both drives are formatted as APFS. I checked a
diskutil output made before the repair work was done (i.e. as it came from the factory), and it seems that both drives were APFS when my iMac was originally built.
Is the APFS format on the HDD in a Fusion Drive a necessity (perhaps due the the need for the SSD using APFS)? If not, could performance/stability be enhanced by using HFS+ on the HDD?
RE: "Has it been reformatted as a Fusion Drive?"
diskutil list output:
necromac:~ seamus $ diskutil list /dev/disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *500.3 GB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_APFS Container disk2 500.1 GB disk0s2 /dev/disk1 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *3.0 TB disk1 1: EFI EFI 209.7 MB disk1s1 2: Apple_APFS Container disk2 3.0 TB disk1s2 /dev/disk2 (synthesized): #: TYPE NAME SIZE IDENTIFIER 0: APFS Container Scheme - +3.5 TB disk2 Physical Stores disk0s2, disk1s2 1: APFS Volume Macintosh HD 139.3 GB disk2s1 2: APFS Volume Preboot 52.0 MB disk2s2 3: APFS Volume Recovery 510.6 MB disk2s3 4: APFS Volume VM 2.1 GB disk2s4
... and diskutil screenshots: