It is unclear whether APFS is not compatible with non-flash memory media is temporary or permanent. In the latest macOS High Sierra test system, the release notes explicitly mention that the iMac computer with built-in 3TB Fusion Drive does not support APFS.
During the testing process, APFS is still compatible with the HFS + file system. Formatted into HFS + file system equipment can be formatted into APFS equipment for data read and write.
Formatted APFS SSD can also be formatted as HFS + file system equipment to read and write, the system requirements for macOS 10.12.6. Only High Sierra can be started through the APFS file format partition, in addition, Boot Camp does not support read and write APFS partition.
Apple APFS file system will be accompanied by the official release of macOS High Sierra and widely deployed, APFS a lot of advantages, including real-time file copy, the overall speed, read and write operations to enhance.