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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.

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

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?

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.

(Note that the createinstallmedia tool keeps the existing partition map flavor of the target disk.)

Apple provides DMG disk images of a few older operating systems. Looking at these, we see that even Apple switched to GPT in 2015.

• OS X 10.10 "Yosemite" - APM
• OS X 10.11 "El Capitan" - GPT
• macOS 10.12 "Sierra" - GPT

Perhaps the use of this flag originates back then. 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.

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

Following identical steps I 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.

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.

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

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.

Apple provides DMG disk images of a few older operating systems. Looking at these, we see that even Apple switched to GPT in 2015.

OS X 10.10 "Yosemite" - APM
OS X 10.11 "El Capitan" - GPT
macOS 10.12 "Sierra" - GPT

Perhaps the use of this flag originates back then. Some decades old blogs mention 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.  Is there actually a specific reason to continue using APM-flavored partition maps for macOS install media?

Following identical steps I created APM- and GPT-flavored install media for macOS Big Sur and tried booting them with VMware Fusion, and only the APM-flavored installer could boot.

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.

(Note that the createinstallmedia tool keeps the existing partition map flavor of the target disk.)

Apple provides DMG disk images of a few older operating systems. Looking at these, we see that even Apple switched to GPT in 2015.

• OS X 10.10 "Yosemite" - APM
• OS X 10.11 "El Capitan" - GPT
• macOS 10.12 "Sierra" - GPT

Perhaps the use of this flag originates back then. 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. 

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

Following identical steps I 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.