Which disk format?

macOS supports several different disk formats, each of which has its purposes. This article explains which to choose from those you can create by formatting a disk in Sequoia. macOS also supports some other formats like NTFS for reading only, which I won’t cover here.

Disk structure

Before any file system can be formatted on a disk, the storage in the disk must be partitioned using one of three standard schemes:

GUID Partition Map (GPT), standard for most disks and file systems in macOS;
Master Boot Record (MBR), formerly used for MS-DOS and Windows;
Apple Partition Map, an old format used by Macs, and worth avoiding unless you know it’s required.

Even if the whole of the disk is going have just one file system or volume, one of those is required, as it stores information about how the space on the disk is allocated.

Volumes and containers

Apart from APFS, most file systems you’re likely to use require a whole partition on the disk for each volume. For example, an HFS+ disk with two volumes is divided into two partitions, each of which contains one HFS+ volume. Because partitioning is intended to be static, that means those two volumes have fixed size, and don’t share free space between them. Once created, it’s possible to change partitioning, and Disk Utility will try to do so non-destructively, without losing any data in the volumes, but that isn’t always possible.

APFS volumes are different, and share free space within a partition, termed a container in APFS terminology. APFS containers are essentially similar to HFS+ volumes, as they’re static partitions, but APFS volumes are sized dynamically within their static container.

Thus, if you have a disk with two HFS+ volumes and two APFS volumes, that disk will have at least three partitions:

one for each of the two HFS+ volumes,
one as the container for the two APFS volumes, although they could instead each be given their own container.

You will see some claims that APFS volumes are more like directories or folders in HFS+, but those are confusing and should be ignored. Each APFS volume has its own file system, and dragging a file from one volume to another results in two completely separate copies of that file, using twice the storage space of one – that’s not how folders behave!

Available formats

Disk Utility version 22.7 in macOS Sequoia 15.0 can format the following file systems using a GUID Partition Map:

APFS, not encrypted and case-insensitive
APFS, encrypted and case-insensitive
APFS, unencrypted and case-sensitive
APFS, encrypted and case-sensitive
HFS+ journalled and case-insensitive (JHFS+)
HFS+ journalled and case-sensitive
ExFAT
MS-DOS (FAT32).

Using a Master Boot Record or Apple Partition Map, the following formats are available:

HFS+ journalled and case-insensitive (JHFS+)
HFS+ journalled and case-sensitive
ExFAT
MS-DOS (FAT32).

APFS is not compatible with Master Boot Record or Apple Partition Map.

The command tool diskutil additionally offers FAT, FAT12, FAT16, Free Space, and HFS+ without journalling, although those aren’t available in Disk Utility.

Note that, contrary to Disk Utility’s Help book, encrypted HFS+ formats are no longer available in Disk Utility, although with the availability of encrypted APFS formats, I can’t think why anyone would want to use encrypted HFS+. That’s because support for encryption was added later to HFS+, whereas it has been designed in from the start of APFS, and is far superior.

Which format for Macs?

The default format for disks to be accessed by Macs is now APFS, not encrypted and case-insensitive, and should be used with all Macs running Mojave and later.

Case-sensitivity can in some circumstances be important. The two situations where this is likely to be required is for the storage of Time Machine backups, and in volumes that might need to store native files from iOS or iPadOS, which use case-sensitive APFS.

Encrypted APFS is different from FileVault used on internal SSDs. However, if you enable FileVault on a macOS installation on external storage, that’s implemented as encrypted APFS. On the internal SSDs of Macs with T2 or Apple silicon chips, FileVault provides additional protection to the keys used for its hardware encryption performed in the Secure Enclave, and should always be enabled. Encrypted APFS is strongly recommended for volumes on external disks that contain private or sensitive data, such as Time Machine backups.

Thus, Time Machine backups should be made to case-sensitive encrypted or unencrypted APFS, and that’s what Time Machine will create for you.

APFS or HFS+?

APFS was designed for use primarily on SSDs rather than hard disks, and doesn’t have features intended to maintain performance when used on hard disks. Unless a volume on a hard disk is intended to store Time Machine backups, it may therefore still be preferable to use either of the supported HFS+ formats. Deciding which is better depends on how that volume will be used.

Because SSDs are largely unaffected by fragmentation of used or free space, APFS isn’t designed to minimise fragmentation, indeed some of its best features inevitably increase fragmentation. In particular, the file system metadata in APFS volumes may become badly fragmented, leading to poor performance on hard disks. Two extreme examples are boot disks and those used to store largely static media libraries.

Recent versions of macOS will happily boot from external hard disks, and despite their relatively poor transfer rates, are far from unusable. Problems come as you use that hard disk, and perform file operations in your Home folder. Over the course of a few weeks or months, fragmentation increases, particularly in file system metadata, and its performance declines.

Media libraries that are most frequently read from, and whose files undergo relatively little change, have fewer changes in their file system metadata, and may well never suffer from noticeable performance impairment. This is also true for Time Machine backups, although some users report deteriorating performance after many months or a year or two.

Time Machine in Sequoia will no longer start a new backup series on HFS+; if you try adding a disk with a single HFS+ volume on it as backup storage, it automatically converts the disk to an APFS container with a single APFS case-sensitive volume, and uses that to store its backups. However, you can still use third-party backup utilities including Carbon Copy Cloner to make backups to HFS+ volumes, although APFS is recommended now. That can have several significant disadvantages, among them:

Snapshots can’t be made of backup storage.
APFS special file types aren’t supported. For sparse files, this can lead to backups being substantially larger than the source files. Worse still, if you restore from a backed-up sparse file, it won’t be automatically converted back to sparse file format on APFS.
Block copying isn’t normally supported, again making backups larger, and increasing the time required to back up.
Where available, incremental backups may become unmanageable because of the number of files and folders.

As a general principle, both APFS and HFS+ can be backed up to APFS backup storage, while HFS+ storage is only suitable for backups from HFS+.

Which format for Windows compatibility?

Although other computers can read HFS+ and sometimes even APFS volumes, few of their users know how to access the Mac’s native file systems. If you need your storage to be accessible from other computers, one of the two supported MS-DOS/Windows formats is best.

Information given in the Disk Utility Help book is slightly misleading over the choice between FAT32 and ExFAT. FAT32 offers a maximum volume size of 2 TB, with files up to almost 4 GB each, but was primarily intended for magnetic media.

Unless the system intending to read the volume is limited to FAT32, it’s generally preferable to use ExFAT instead. That supports massive volumes and file sizes, and was optimised for use in flash memory. ExFAT is thus the more commonly encountered format for USB flash drives (thumb drives, memory sticks) and SD cards, where it’s the default format for SDXC and SDUC cards larger than 32 GB.

FAT32 and ExFAT are supported with both GUID Partition Map and Master Boot Record schemes. Unless the other computer or system is very old, prefer GUID Partition Map. Neither volume format supports encryption, so if you want to protect files you should encrypt them separately, or store them in an accessible encrypted archive format.

Summary

For general purposes, default disk format should use a GUID Partition Map with either plain or encrypted APFS.
Encrypted APFS should be used for volumes on external disks containing private or sensitive data. File Vault should be enabled on internal SSDs.
New Time Machine backups can now only be made to case-sensitive APFS, either plain or encrypted.
Hard disks with active file systems may suffer poor performance with APFS, and HFS+ with journalling may still be preferable, but it has significant limitations and disadvantages.
Hard disks for more static use, such as for media libraries and backups, should be safe with APFS, but may eventually suffer poor performance.
Unless there are good reasons for using FAT32, format USB flash drives, SDXC and SDUC cards that need to be used with non-Mac systems using ExFAT in a GUID Partition Map scheme.