So what happens to this partition after you are done installing? During the next boot into OS X, the OSXRESERVED partition is removed and put back into the Core Storage container of the OS X partition: It appears that newer Macs have the ability to detect this partition and present it to Windows as if it were EFI installation media (such as a DVD or USB Flash drive).

The EFI folder on the OSXRESERVED partition is the same one you would normally find on the EFI partition (normally disk0s1). If you are familiar with EFI booting on OS X, you’ll see a familiar setup. The OSXRESERVED partition has all the installer files, the Boot Camp drivers for Windows, and the EFI files for booting. You’ll also notice that disk2 is the Windows install ISO disk image that the Windows install files are copied from. Partition 4 is now the OSXRESERVED partition, and partition 5 is the BOOTCAMP partition. Partition 1 is the standard EFI partition, partition 2 is the Mac partition, partition 3 is the Recovery partition. It places this partition right after the recovery partition, and before the Boot Camp partition, as shown below. So how is this possible? Where is the Windows installer if there is no installation media? Boot Camp Assistant doesn’t just create a Boot Camp partition, but also creates an additional partition called “OSXRESERVED” that is FAT32 formatted. Just select the ISO and how much space you want from Boot Camp, and then you click Install.Īfter Boot Camp Assistantd completes, OS X restarts to the Windows installer, and you follow the normal Windows installation.

Prior to El Capitan, you had to insert a USB Flash Drive and Boot Camp Assistant copied the Windows installer from an ISO disk image to the flash drive, and then downloaded and set up the Windows drivers to the correct location in the installer for the Mac hardware. Notice the ISO image and partitioning are all on a single screen. When you open Boot Camp Assistant on a new Mac that supports Windows 8 or later, you’ll get the new Boot Camp interface. Some very interesting changes in how Windows is installed in Boot Camp on OS X 10.11 “El Capitan”.

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