APFS file format read-write compatibility

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Devices formatted as Mac OS Extended (HFS+) can be read from and written to by devices formatted as APFS.
Devices formatted as APFS can be read from and written to by:
Other devices formatted as APFS
Devices formatted as Mac OS Extended, if using macOS High Sierra.
For example, a USB storage device formatted as APFS can be read by a Mac using High Sierra, but not by a Mac using Sierra or earlier.
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If you do NOT have an SSD drive in your Mac your drive format will be unaffected by the High Sierra upgrade. This includes traditional "spinning metal" and "Fusion" drives.