macOS High Sierra Makes APFS the Default Filesystem

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.

iOS got there first

APFS didn’t debut in High Sierra — Apple had already shipped it as the default filesystem across iOS, tvOS, and watchOS with iOS 10.3, released in March 2017, six months ahead of the Mac conversion. That sequencing was deliberate: iOS’s more constrained, more uniform hardware and software environment gave Apple a lower-risk proving ground to validate APFS at genuinely enormous scale (hundreds of millions of devices) before trusting it with the Mac’s much more heterogeneous hardware, third-party filesystem tooling, and enterprise deployment scenarios — a real-world stress test most filesystem transitions never get the chance to run before their highest-stakes rollout.

Hard drives and Fusion Drives caught up two years later

The Mac’s hybrid and spinning-disk holdouts didn’t stay on HFS+ forever — macOS Catalina, released in 2019, extended automatic APFS conversion to Fusion Drives and traditional hard drives as well, once Apple’s engineering had addressed APFS’s original performance characteristics on non-flash media. By that point, the vast majority of actively-sold Macs already shipped with flash storage exclusively, making the Catalina extension more a completion of the transition for a shrinking population of older machines than a change most current Mac buyers ever directly experienced — the High Sierra rollout in 2017 had already done the heavy lifting for the overwhelming majority of the active installed base by the time Catalina finished the job for everyone else two years later, leaving only the oldest spinning-disk holdouts still waiting on the older HFS+ format by that point.

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