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macOS High Sierra introduces a new file system call Apple File System, or APFS. APFS replaces the old file system, called HFS+. APFS has a lot of inherent advantages beyond just being modernized that improve reliability and speed. Chances are, however, that you have external hard drives and thumb drives formatted using HFS+.

Reader Ted Tenny wondered if thumb drives formatted using a macOS prior to High Sierra will continue to work in High Sierra, which automatically converts startup SSDs to APFS.

The answer is: yes, absolutely. Apple doesn’t require that you change the file system format on external drives, whether attached at startup or plugged in temporarily. You can use Disk Utility in High Sierra to upgrade to APFS, but you will probably not want to do that with any drive that you might want to use with a Mac that isn’t yet running High Sierra.

Apple could at some point remove HFS+ support, but that would only come some years in the future, when there’s no longer any sense in backwards compatibility.

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