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I read things such as:

  • Mac OS X may be unstable with more than sixteen partitions
  • up to sixteen partitions on a external drive.

Also I understand that in a sixteen-slice environment, the full installer for OS X Mountain Lion may be unable to add an Apple_Boot slice for Recovery OS.

If there's a limit

Why the number 16?


References

Red Hat Bugzilla Bug 460390 – Can't make apple boot partition catch 22 during install (netinstall CD): comment 8 (2008-11-15)

… more than 16 partitions on the drive … Mac OS X did boot with the whole drive formatted (as several partitions), or with more than 16 partitions, but it did not recognize the entire drive, and may not have been stable. …

.Drives, partitions, formatting w/Mac's + PC's: Apple Support Communities (2012-12-05)

… up to 16 partitions on a external drive …

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1 Answer

A maximum of sixteen partitions per GPT disk may be for one or both of the following:

  • compatibility with other BSD variants
  • UNIX certification/conformance.

Key points

In theory, a GUID Partition Table (GPT) disk can have an unlimited number of partitions.

A Microsoft implementation of GPT allows a maximum of 128 partitions.

The OS X kernel, libraries, and utilities are drawn from a number of open source projects, including FreeBSD, GNU, NetBSD, and OpenBSD.

Some BSD variants have a maximum of sixteen partitions.

Sometimes misunderstood

Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits

Volume and file limits in Mac OS X

Maximum number of volumes (all Mac OS X versions): no limit …

That is:

  • volumes per operating system
  • not partitions per disk.

Expect the operating system to handle:

  • thirty-two or more volumes spread across two or more disks
  • and so on.

Further reading

disklabel(8) OS X Manual Page (2004-09-03)

Frequently asked questions about the GUID Partitioning Table disk architecture (2009-08-04)

…The Microsoft implementation of GUID Partition Table is limited to 128 partitions. …

OS X Glossary (2010-07-09)

BSD Berkeley Software Distribution. Formerly known as the Berkeley version of UNIX, BSD is now simply called the BSD operating system. The BSD portion of Darwin is based on 4.4BSD Lite 2 and FreeBSD, a flavor of 4.4BSD.

Unix 03 Conformance Release Notes (2010-09-01)

… FreeBSD, GNU, NetBSD, and OpenBSD … make an effort to follow industry standards …

www.bsd.org (2011-03-30)

How-To Dual Boot FreeBSD, Linux, and OpenBSD with GNU Grub (undated but Copyright © 2000 - 2012)

… OpenBSD's disklabel allows up to 16 partitions. …

Register of Open Branded Products: five product standards for which Mac OS X Version 10.8 Mountain Lion is registered (2012-07-10)

BSD disklabel – Wikipedia (2013-01-27)

… BSD disklabels traditionally contain 8 entries for describing partitions. These are, by convention, labeled alphabetically, 'a' through to 'h'. Some BSD variants have since increased this to 16 partitions, labeled 'a' through to 'p'. …

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