'Boot OSX' folders appearing in OS X

Sometimes people who have just reinstalled OS X on their systems or migrated to a new boot device may find a folder entitled "Boot OSX" appearing on their desktops.

Topher Kessler MacFixIt Editor
Topher, an avid Mac user for the past 15 years, has been a contributing author to MacFixIt since the spring of 2008. One of his passions is troubleshooting Mac problems and making the best use of Macs and Apple hardware at home and in the workplace.
Topher Kessler
4 min read

Sometimes people who have just reinstalled OS X on their systems or migrated to a new boot device may find a folder entitled "Boot OSX" appearing on their desktops. The folder cannot be removed, and only seems to disappear when the system is restarted. In a nutshell, these folders appear because of a glitch that recognizes a residual boot helper partition that is used on non-standard filesystem setups.

On nonstandard filesystems such as RAID disks (including those used in an Apple Software RAID setup through Disk Utility), the system will create the main storage partitions, but in addition it will create a small partition called "Boot OSX" in which it stores the boot loader for OS X along with a copy of the system kernel and its configured extensions in a multi-extensions cache (mkext cache).

On a software RAID, the member disks each have a 128MB boot helper partition called "Boot OSX". In this example the two disks (disk1 and disk2) are combined to create "disk3" in a mirrored RAID array (click for larger view).

When it comes to loading the OS X kernel, the fastest boot will happen if the system uses the prelinked kernel, which is a cache file that contains the kernel code that is already linked up with its kernel extensions and other configuration files. This file is located in the /System/Library/Caches/ folder and is called "com.apple.kernelcaches."

The second fastest option is to use the mkext cache, which contains all the compressed extensions that the current system configuration uses. If there are problems or inconsistencies in the prelinked kernel then the system will resort to using the mkext cache at boot.

The last option is for the system to load the kernel and extensions individually from their locations on disk, which is the slowest way to load and configure the system kernel. This requires the EFI firmware and boot loader to locate the kernel on disk, then load each of the used kernel extensions into the kernel, which can take significantly longer than using a cached kernel and extensions.

In an OS X installation on a normal hard disk, the multi-extensions cache and boot loader are located on disk in the following directories:

Multi-extension cache (one of these two locations):

Boot Loader:

When the Mac hardware is first started up, the EFI firmware will become active and load. EFI is file-system-aware when it starts up, allowing the firmware to load files directly from disk. Normally the system will access the prelinked "kernelcache" or boot loader and kernel extension caches from their respective locations on the boot drive; however, in situations where the boot volumes are set up in a nonstandard way (e.g., RAID sets, case-sensitive file systems, or other non-standard file systems like UFS) the system will create helper partitions to hold the mkext and boot loader files. These helper partitions will be 128MB in size and be labeled "Boot OSX." Their sole purpose is to provide the system quick access to the boot files needed for starting the system in the event the prelinked kernel cannot be used.

If you are not using a RAID array or other special setup for the boot disk, you do not need to have the Boot OSX partition. As a result you can clear this problem but it will require you to repartition your boot drive. To do this, you will need to either backup your system using Time Machine and then partition the drive and restore your data, or perform a similar task by cloning your system.

Time Machine:

  1. Make a full backup of your system using Time Machine and then unmount and disconnect the Time Machine drive.

  2. Boot to the OS X installation DVD, select your language, and then select "Disk Utility" from the "Utilities" menu.

  3. Click your boot drive device (the item above the "Macintosh HD" volume name) and click the "Partiiton" tab.

  4. From the "Volume Scheme" menu select "1 Partition," then name the volume and format it to "Mac OS Extended (Journaled)."

  5. Quit Disk Utility and choose "Restore System from Backup" from the "Utilities" menu.

  6. Follow the onscreen instructions to restore the latest Time Machine backup back to your repartitioned boot disk.

System Clone:

  1. Obtain a spare external drive, and partition it following steps 3 and 4 listed above.

  2. Using a cloning tool of your choosing, make a file-level clone (as opposed to "block-level") of your boot drive to a spare hard disk. Be sure to test the clone by booting off it (hold the Option key at boot to select it).

  3. Boot the system to the cloned drive, and open Disk Utility.

  4. Follow steps 3 and 4 in the "Time Machine" instructions to partition the internal boot disk.

  5. Quit Disk Utility and open the cloning tool.

  6. Clone your OS installation back to the internal drive (you can use either file-level or block-level cloning at this point).

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