Volume structure of internal startup disks has grown increasingly complex during the transition from Intel to Apple silicon Macs. There also seems to be little information on iOS and iPadOS to compare against. This article briefly reviews structures of macOS 15 Sequoia on Apple silicon, iOS 18 and iPadOS 18.

Information for macOS is derived from the diskutil command tool, and from APFS entries in the log when booting a Mac mini M4 Pro in macOS 15.5. That for iOS is drawn from APFS entries in the log when booting an iPhone 15 Pro in iOS 18.5. That for iPadOS is drawn from APFS entries in the log when booting an iPad Pro 11-inch (4th generation)(Wi-Fi) in iPadOS 18.5. All three had Apple Intelligence enabled prior to booting. iOS and iPadOS logs were obtained from sysdiagnoses, and all logs were read using LogUI.

macOS 15 (Apple silicon)

The boot volume group consists of six volumes in a single container (partition). Two other containers are normally hidden from the user:

• the first container of around 524 MB is reserved for preboot and secure boot support.
• another container of about 5.4 GB is used for fallback recovery frOS, and in Big Sur was the primary recovery system, until the introduction of paired recovery volumes in macOS 12 Monterey.

The boot volume group contains:

• System, left unmounted after booting from its Signed System Volume (SSV) snapshot;
• Data, the only encrypted volume in the group, with numerous cryptexes grafted into it, and firmlinked to the SSV at multiple points;
• paired, primary Recovery, containing a disk image of the Recovery system;
• VM, the backing store for virtual memory;
• Preboot, for early stages in the secure boot process, with cryptexes grafted into it;
• Update, used as a working volume for macOS updates.

There are two groups of cryptexes grafted onto those volumes:

• system cryptexes, including the large SystemCryptex or os.dmg of about 4.3 GB mainly containing dyld caches, and the smaller AppCryptex or app.dmg containing Safari and supporting components;
• PFK volumes containing support components for Apple Intelligence features. These are numerous, and some are listed in the Appendix at the end.

If you’re wondering what a PFK volume might be, so am I. But this is what Google’s AI had to say: “Mac PFK” likely refers to a combination of MAC knives and Practical Fishkeeping (PFK) magazine. MAC knives are known for their high-quality, sharp blades and are popular among chefs and home cooks. Practical Fishkeeping is a magazine focused on fishkeeping, covering various aspects of the hobby.

These are summarised, without the help of Practical Fishkeeping, in this diagram.

iPadOS 18

In contrast to macOS since Big Sur, iPadOS and iOS only appear to have two containers (partitions) on their internal storage. The first is presumed to be similar in purpose to that in macOS, in supporting preboot and secure boot, although there is a xART volume in the boot volume group. In iPadOS, this container is smaller, at around 367 MB.

The boot volume group contains a slightly different range of volumes:

• there is no Recovery volume;
• there is no VM volume, as iPadOS doesn’t ordinarily support swapping/paging, although M-series models can in certain circumstances;
• User, a second encrypted volume, appears unique to iPadOS;
• xART and Hardware volumes are additional.

Cryptexes appear similar, with both system cryptexes and PFK volumes.

These are summarised below.

iOS 18

This is similar to iPadOS, with a first container/partition of around 351 MB, and the following differences in the boot volume group:

• there is no User volume, and no Update volume;
• Baseband Data is additional.

Cryptexes appear similar, with both system cryptexes and PFK volumes.

These are summarised below.

Conclusions

• Volume structure of internal startup disks differs considerably between macOS, iPadOS and iOS.
• As would be expected, iPadOS and iOS are most similar, but even they have substantial differences.
• They each run their systems from a Signed System Volume firmlinked to an encrypted Data volume.
• They each graft on two sets of cryptexes, one supplementing the system with dyld caches and Safari, the other providing components for AI.
• There are now at least 24 cryptexes used to support AI.

I welcome corrections and explanations, please.

Appendix: Some PFK volumes from cryptexes

• UC_FM_LANGUAGE_INSTRUCT_300M_BASE_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_300M_BAUC_FM_LANGUAGE_INSTRUCT_300M_BASE_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_DRAFTS_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_CONCISE_TONE_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_MAIL_REPLY_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_MAIL_REPLY_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_BASE_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_PROFESSIONAL_TONE_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_SUMMARIZATION_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_TEXT_EVENT_EXTRACTION_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_PROOFREADING_REVIEW_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_HANDWRITING_SYNTHESIS_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_TEXT_EVENT_EXTRACTION_MULTILINGUAL_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_MESSAGES_REPLY_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_AUTONAMING_MESSAGES_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_URGENCY_CLASSIFICATION_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_FRIENDLY_TONE_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_HANDWRITING_SYNTHESIS_MULTILINGUAL_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_VISUAL_IMAGE_DIFFUSION_V1_BASE_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_TEXT_PERSON_EXTRACTION_MULTILINGUAL_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• SECUREPKITRUSTSTOREASSETS_SECUREPKITRUSTSTORE_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_TEXT_PERSON_EXTRACTION_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_FM_LANGUAGE_INSTRUCT_3B_MAGIC_REWRITE_DRAFT_GENERIC_GENERIC_H14G_Cryptex.dmg
• UC_IF_PLANNER_NLROUTER_BASE_EN_GENERIC_H14G_Cryptex.dmg

Source: iPadOS 18.5, configured with AI enabled for British English.