macOS High Sierra Makes APFS the Default Filesystem
Released September 25, 2017, High Sierra automatically converted flash-storage Macs to Apple File System — the biggest filesystem transition on the Mac in nearly two decades.
macOS 10.13 High Sierra shipped on September 25, 2017, and with it came the biggest filesystem change to the Mac in close to two decades: automatic conversion of eligible startup disks to APFS, Apple’s modern filesystem, replacing the decades-old HFS+.
Which Macs converted, and which didn’t
Any Mac with flash storage as its built-in boot drive was converted to APFS automatically during the High Sierra installation — no user choice involved. Macs using traditional hard drives or Fusion Drives (a hybrid of flash and spinning disk, common in iMacs of that era) were explicitly not converted, since APFS at that point was optimized specifically for flash storage characteristics.
An unusually invisible major migration
Converting a live, in-use boot filesystem to an entirely different on-disk format, silently, as part of an ordinary OS upgrade, without data loss or requiring a separate backup-and-restore step, was a genuinely difficult engineering achievement — most filesystem transitions of this scale in computing history have required an explicit reformat. Apple’s migration tooling handled the conversion as an integrated part of the installer itself.
Opting out, if you needed to
For users or administrators who needed to defer the conversion — third-party disk utilities or backup tools not yet updated for APFS, for instance — the installer’s startosinstall command line tool supported a --converttoapfs NO flag, giving enterprise deployments a way to delay the transition until their tooling caught up.
Why this milestone mattered
APFS brought space-efficient snapshots, native encryption, and clone files as first-class filesystem features — capabilities covered in depth elsewhere on this blog that directly enable things like Time Machine’s local snapshots today. High Sierra’s automatic conversion is the point those capabilities became the default reality for the vast majority of the Mac installed base, rather than an opt-in feature for advanced users.
Sources: Apple File System — Wikipedia, Using the macOS High Sierra OS installer’s startosinstall tool to avoid APFS conversion — Der Flounder