Why use the legacy Apple Partition Map scheme for macOS install media?

Question

Several guides to creating disk images of macOS install media (using the createinstallmedia command) recommend creating the disk image like this:

hdiutil create \
    -size 10g \
    -type UDTO -layout SPUD -fs JHFS+ \
    ~/Desktop/installer.cdr

Here, -layout SPUD creates the disk image with an Apple Partition Map, rather than the default, more modern GUID Partition Table.

According to the hdiutil manpage,

SPUD causes a DDM and an Apple Partition Scheme partition map with a single entry to be written. GPTSPUD creates a similar image but with a GUID Partition Scheme map instead. When attached, multiple /dev entries will be created, with either slice 1 (GPT) or slice 2 (APM) as the data partition. (e.g. /dev/disk1, /dev/disk1s1, /dev/disk1s2).

Unless overridden by -fs, the default layout is GPTSPUD (PPC systems used SPUD prior to Mac OS X 10.6). Other layouts include MBRSPUD and ISOCD. create -help lists all supported layouts.

Some decades old blogs also note that GPT disks are not compatible with PowerPC Macs, and very old versions of macOS (prior to 10.4.2) do not understand GPT partition maps at all. And years ago there was briefly an incompatibility with VirtualBox when using GPT-formatted install media. So perhaps APM was considered a more broadly compatible format. But these are just theories.

Note that the createinstallmedia tool keeps the existing partition map flavor of the target disk, and Apple's KB articles indicate no preference for one over the other. The default when formatting a disk with Disk Utility in macOS Monterey is GPT, though, so this is likely what most people use.

(As an experiment, I followed identical steps to created APM- and GPT-flavored install ISOs for macOS Big Sur and tried booting them with VMware Fusion, and only the APM-flavored installer could boot. I'm investigating this further.)

Is there actually a specific reason to continue using APM-flavored partition maps for macOS install media?

Answer 1

I believe the answer depends on how you intend to use the ISO.

Use GPT when you want an ISO file that can be written to a bootable USB thumb drive.

Use APM when you want to install macOS on a virtual machine by attaching the ISO file to an emulated CD/DVD drive.

You could also use APM on a physical drive as long as the machine's UEFI understand that format.

And you probably can use GPT install media on a virtual machine by attaching it as a virtual hard drive, but you likely have to convert it to another format than ISO, such as VMDK.

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As I understand it (details welcome in comments) CD/DVD drives typically use a 2048-byte sector size, whereas macOS's disk images can only be created with a 512-byte sector size. Therefore there's a sector size mismatch when attaching the ISO through an emulated CD/DVD drive, but the firmware knows that APM is always 512 bytes and is able to understand it correctly.

(I think GPT can support other block sizes, but there's no way to control this on macOS.)