Apfs Or Mac Os Extended For Imac Hdd High Sierra

9 min readMay 26, 2021

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Apple migrated its users to a new, modern file system called Apple File System alongside the release of macOS High Sierra 10.13 back in September of 2017, aiming to take advantage of flash and SSD storage drives, almost primarily because of the fact that all new Apple products use SSD as the storage disk.

  • Some Macworld readers are concerned about upgrading to macOS High Sierra, which offers the new APFS (Apple File System) that replaces the nearly 20-year-old HFS+ filesystem.
  • 2) Mac OS Extended (HFS Plus) disks can be rebuilt as before. 3) The internal drives of Macs are automatically converted from Mac OS Extended (HFS Plus) to Apple File System (APFS) when upgrading to macOS 10.14 Mojave. Only internal SSD drives are converted to APFS by macOS 10.13 High Sierra.

This file system is specifically designed for Apple products but that doesn’t necessarily mean that other files systems are bad, just that they serve other purposes. It was announced at Apple’s Worldwide Developers Conference in 2016 as a replacement for the then default Hierarchical File System Plus (HFS+) file system. Apple released APFS (Apple File System) for mobile devices on March 27, 2017, along with the release of iOS 10.3.

Apfs Or Mac Os Extended For Imac Hdd High Sierra Mac

Migrating from HFS to APFS

APFS replaces HFS+ (also known as Mac OS Extended), Apple’s proprietary file system that had been used for decades and which builds on the original HFS, referred to as Mac OS Standard. HFS+ was designed in an era when floppies and hard drives were the best storage technologies. But since then a lot has changed: almost all Apple products use SSDs, and the next-generation storage technology is continually evolving.

My Macbook Air has SSD so it automatically switched to APFS when I updated to High Sierra. I don’t recall being asked if I wanted to when I updated an older iMac to High Sierra. I was surprised to find it is Mac OS Extended and not HFS+. Either way I did a bunch of reading and APFS is tuned for SSD but I’m still wondering about updating for an HDD.

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APFS Key Features: Encryption

The new APFS was developed with encryption at its core, likely due to Apple’s requirements of using the same file system across various devices; consider multiple user keys within the file system on iPhone or iPad, or single-user keys on portable computers. According to Apple’s documentation, APFS supports the following encryption models for each volume in a container:

  • Unencrypted
  • Single-key encryption
  • Multi-key encryption with per-file keys for file data and a separate key for sensitive data.

The latter is particularly important for fully encrypted portable devices, but unlocking the iPhone or iPad gives the user access to additional keys and, as such, additional data.


By taking snapshots of the whole file system at a certain point in time, macOS essentially freezes a point in time that it can always revert back to. Every change made in the file system is then tracked and compared to that captured moment, and only new data takes up additional space. Essentially, it creates a read-only, independently mountable clone of the file system and saves the changes in a new location to preserve the integrity of that clone.

Space Efficiency

How many times have you struggled to optimize your Mac’s storage by locating duplicate files and folders? The new file system now allows users to store data more efficiently, so if you copy a file to a new folder on the same computer, no data is actually duplicated. What happens instead is that the metadata is updated and the on-disk data is shared between locations. Changes in the ‘copy’ will trigger new space allocation, called copy-on-write, which also ensures that updates to the file system are crash protected.

While you might wonder why someone would create a copy of the file on the same computer, the average user might be tempted to keep a copy under a different name, such as “presentation-draft,” “presentation-1,” “presentation-final,” or “presentation-typo” when working on a Keynote presentation. Users working with larger files — such as video editors — might also see APFS’ space efficiency as a huge advantage.


Since APFS was developed for flash drives, it includes TRIM support. With this new file system Apple is also focusing on avoiding frustration among users caused by the annoying beach ball of doom. To do that, Apple uses I/O QoS (quality of service), which has the system arrange tasks into priorities, and obviously focuses on resolving the processes that are immediately important.

Apple applies a three-point protection method to ensure data integrity:

  • All SSDs and hard drives used in Apple products use Error Correcting Code (ECC) to check for transmission errors, and corrects them.
  • APFS uses the copy-on-write scheme.
  • APFS uses the Fletcher’s checksum algorithm for metadata operations.

The Negatives of APFS

Though there are a lot of benefits of using APFS, there are some drawbacks in making the switch from HFS+. While it is mandatory on Macs running High Sierra 10.13 or above — though the conversion process can be skipped with the right know how — it’s worth being aware of what will change with the new file system. The negative points of APFS are:

  • It doesn’t provide checksums of user data.
  • It doesn’t take advantage of byte-addressable, non-volatile random-access memory.
  • It doesn’t support compression.
  • It doesn’t support deduplication.


With APFS Apple moved away from a file system it had used for the previous 30 years, HFS, which was introduced in 1985. Apple improved the original HFS in 1998 with HFS+ by adding support for much larger files and Unicode for naming files. It also utilized a full 32-bit allocation mapping table instead of HFS’s 16-bits, increasing the allocation blocks to four billion. Despite its welcome benefits, the HFS+ lacks features that are considered highly important in modern file systems, including: data checksums, nanosecond timestamps, snapshots, support for dates beyond February 6, 2040, sparse file support, and better implementation of hard links.

APFS aims to fix these core issues by using 64-bit inode numbers, therefore enabling better space allocation and supporting over nine quintillion files on a single volume. It also increases read-write speeds on both iOS and macOS devices, as well as the available space on mobiles due to the way that APFS calculates the available data.


Developed by Microsoft, the NTFS (New Technology File System) is shipped as the default of the Windows NT family. While the maximum number of files matches that of HFS+, NTFS has several advantages over Apple’s previous default file system: it supports file compression and data deduplication, among other features.

For some reason, interoperability between the NTFS, HFS+ and APFS file systems isn’t smooth: Windows computers can only read NTFS formatted drives, while macOS can read an NTFS formatted hard drive but it cannot write to it. This gap is closed by third party drivers such as Paragon NTFS for Mac, which enables writing to an NTFS-formatted disk. However, Mac users usually refrain from using this file system unless they are working with a Windows machine.

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Summing up

It’s easy to understand how building a new file system was cheaper than spending time and money on maintaining and evolving a 30-year-old one. Yet despite the improvements, the new file system still has to mature, and Apple is apparently doing everything possible to speed the process up, especially since a file system can take a decade to mature. Still, APFS seems like a work in progress, because at this point it doesn’t fully support HDDs and Fusion Drives. It also brings with it new problems, such as when partitioning a volume; the Time Machine’s local snapshots feature doesn’t allow partitioning unless these snapshots are disabled.

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Disk Utility User Guide

Partitioning a disk divides it into individual sections, each of which acts as a separate volume.

However, with APFS, you shouldn’t partition your disk in most cases. Instead, create multiple APFS volumes within a single partition. With the flexible space management provided by APFS, you can even install another version of macOS on an APFS volume.

Important: If you’re partitioning your internal physical disk because you want to install Windows, use Boot Camp Assistant instead. Do not use Disk Utility to remove a partition that was created using Boot Camp Assistant. Instead, use Boot Camp Assistant to remove the partition from your Mac.

Add a partition

Important: As a precaution, it’s best to back up your data before creating new partitions on your device.

  1. In the Disk Utility app on your Mac, select a volume in the sidebar, click the Partition button , then click Partition.
  2. Internal storage devices appear below the Internal section in the sidebar. External devices appear below the External section in the sidebar.
  3. When you select a volume that already has data on it, the pie chart shows a shaded area representing the amount of data on the volume and an unshaded area representing the amount of free space available for another volume. Disk Utility also shows whether the volume can be removed or resized.
  4. Note: If you see a small volume with an asterisk, the partition is smaller than can be represented at the correct scale in the chart.
  5. Click the Add button .
  6. Type a name for the volume in the Name field.
  7. For MS-DOS (FAT) and ExFAT volumes, the maximum length for the volume name is 11 characters.
  8. Click the Format pop-up menu, then choose a file system format.
  9. Enter the size or drag the resize control to increase or decrease the size of the volume.
  10. Click Apply, click Partition, then click Continue.
  11. Click Show Details to view the step-by-step process of creating a new volume.
  12. After the volumes are created, click Done.

After you partition a storage device, an icon for each volume appears in both the Disk Utility sidebar and the Finder sidebar.

Delete a partition

WARNING: When you delete a volume or partition, all the data on it is erased. Be sure to back up your data before you begin.

  1. In the Disk Utility app on your Mac, select a volume in the sidebar, click the Partition button , then click Partition.
  2. In the pie chart, click the partition you want to delete, then click the Delete button .
  3. If the Delete button is dimmed, you can’t delete the selected partition.
  4. Click Apply, then click Partition.
  5. After the volume is deleted, click Done.

Erase a partition

  1. In the Disk Utility app on your Mac, select the volume you want to erase in the sidebar.
  2. Click the Erase button , then click Erase.
  3. If the Erase button is dimmed, you can’t erase the selected volume.
  4. After the volume is erased, click Done.

Enlarge a partition on a storage device

If you have multiple partitions on a device and one of them is running out of space, you may be able to enlarge it without losing any of the files on it.

To enlarge a volume, you must delete the volume that comes after it on the device, then move the end point of the volume you want to enlarge into the freed space. You can’t enlarge the last volume on a device.

WARNING: When you delete a volume or partition, all the data on it is erased. Be sure to back up your data before you begin.

High Sierra Apfs Or Mac Os Extended

  1. In the Disk Utility app on your Mac, select a volume in the sidebar, then click the Partition button .
  2. In the pie chart, select the partition you want to delete, then click the Delete button .
  3. Click Apply.
  4. The partition is removed, reformatted, and all free space is assigned to the previous partition.
  5. Click Done.

See alsoFile system formats available in Disk Utility on MacAdd, delete, or erase APFS volumes in Disk Utility on MacAbout Disk Utility on Mac

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