Partitioning GPT Disks for Bootcamp

by dhovel 12. May 2013 16:52

After some learning, here are some notes about partitioning large, external GPT disks for use on Macs running Bootcamp.

Disks larger than 2 terabytes must be organized as GPT (Guid Partition Table) disks. This newer structure not only allows disks up to 9 TB to be used but also allows direct specification of several partitions without "extended partition tables".  GPT-based partitions, however, are more complex and require "guard" partitions of empty disk space and "dummy" MBR information.  Both Windows and OSX can create and deal with these disks, but strange things can happen that prevent correct allocation.

I found that OSX had the easiest method for partitioning a GPT disk.  The "Disk Manager" in OSX allows a choice of from 1 to 16 partitions, along with a "drag" slider to apportion the space.  I tried to use this to create a dual-partition disk with one partition for the Mac (HFS+ Journalled) and one for Windows (NTFS).  I rebooted to Windows to convert the larger partition to NTFS.

This seemed to work, and both partitions were recognized on my Macbook under both Windows and OSX. However, when moving the external drive to another Windows machine, the other Windows would not recognize the NTFS partition.  I verified that only my Macbook with the Bootcamp drivers would correctly recognize the NTFS partition.

Then I tried the same thing with the "Disk Management" tool under Windows. The problem here was that after rebooting to OSX, the Disk Manager would not allow me to reformat the smaller partition as HFS+; I kept receiving an "insufficient space" error.

Finally I was able to accomplish this under Windows.  Using Disk Managment, I removed all partitions on the disk.  Then I allocated a large NTFS partition and a smaller one as NTFS (destined later to be HFS+), but I left a sizable "void" of unallocated space beyond the smaller partition.  Rebooting to OSX, I was able to convert the smaller partition to HFS+.  The resulting NTFS partition is recognized by other (non-Bootcamp) Windows machines.

It appears that 1) the Bootcamp drivers allow Windows to recognize disk configurations that other (non-Bootcamp) Windows machines do not, and 2) the OSX Disk Manager has a bug.  It will not reduce the space of a partition to provide for the "guard" partition that GPT allocation requires.  By leaving unallocated (empty) space beyond the second, smaller partition, the Disk Manager was able to carve the "guard" partition from the unallocated space and correctly erase (reformat) the partition for HFS+.

The only remaining problem is that the Bootcamp drivers in Windows don't recognize the GPT-based HFS+ partition; I keep getting a message that "the disk in drive X: must be formatted for use in Windows".  I just ignore this error and everything seems fine.

YMMV.  Good Luck.

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