High Sierra's APFS optimized for flash storage & SSD, incompatible with legacy HDDs and possibly Fusion Drives

The new APFS brings notable enhancements to High Sierra and iOS, but on the Mac it appears that legacy hard drives are completely incompatible with the technology and at least at launch that Fusion drives may not be either.

In an Apple support document published on Aug. 21, the company notes that systems with flash and SSD storage will be configured automatically — and users cannot opt-out of the transition. However, in the same document, it also notes that systems with hard drives and Fusion drives won't be converted at all.

During the early beta process, it was not mandatory to migrate flash-based system drives to APFS.

It is unclear if this is a permanent state for APFS in regards to non-flash based storage media. In the latest developer beta of High Sierra, the hard drive prohibition is clear, but release notes declare that "some" iMacs with 3TB Fusion Drives "may be unsupported for use with APFS."

During the testing process, compatibility remains with HFS+ in APFS. Devices formatted as HFS+ can be read and written to by devices formatted as APFS, and drives formatted as APFS can be read and written by HFS+ formatted systems running macOS 10.12.6 or later.

Only High Sierra can boot from an APFS-formatted partition, however. Additionally, Boot Camp does not support reading to or writing from APFS volumes.

AppleInsider has reached out to Apple for clarification of the discrepancies between the support document and the release notes for the High Sierra beta, and will update with any information that is shared with us.

Apple's next-generation APFS made its way to macOS High Sierra after an official debut on iOS 10.3. With it comes essentially instant file copies, better efficiency for greater overall speed, and fine-tuning of read and write operations boosting system performance.

Early AppleInsider testing of APFS has shown notable advances in speed and gains in storage.

Prior to installation, the PCI-E SSD on a 2015 i7 MacBook Pro booted in about 24 seconds from a shut-down machine. Following installation, the boot time from "cold iron" was cut to 18 seconds — 75 percent of the previous time.

The results on the SATA-3 SSD in a 2012 i7 Mac mini were more dramatic. A boot from a fully shut down state normally took about 42 seconds. Following High Sierra installation, boot times dropped to 64 percent of that in Sierra, taking only 27 seconds.

Prior to downloading the installer on our two test beds for High Sierra, the MacBook Pro had 222 GB used for the system, applications, and a large number of documents with nearly no duplications. Following installation, the MacBook Pro shrunk to 202 GB used — nearly a 10 percent savings.

A more recent installation on a 2016 15-inch i7 MacBook Pro shrunk from 261GB to 235GB, delivering a very similar result percentage-wise to the older MacBook Pro.