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As I have spent hours searching for solutions to install a third data partition, accessible by both Mac OS X and Windows XP installed on an Intel Machine, I want to share my experience and publish this Guideline:
Guideline Background
Both Mac OS X and Windows can read and write to partitions formatted with FAT and FAT32. Unfortunately, Mac OS X can read but not write to NTFS formatted partitions that Windows NT, 2000, and XP can use instead of FAT32. NTFS should be used for a Windows partition, as it is safer.
A limitation when it comes to partitions is that the Windows MBR can address 4 primary partitions only. The first 2 partitions on Intel Macs are reserved for something called EFI (200 MB, normally invisible) and for the Mac OS itself. That leaves room for 2 additional partitions, one for your shared “bilingual” data and one for Windows XP.
For a strange reason, Windows XP MUST be installed on the LAST partition in order for the EFI boot loader to recognize it (“hal.dll corrupt or missing” or “missing media” error).
So the recommended solution is:
First Partition (i.e. “disk0s1”): reserved EFI partition (invisible in Disk Utility and during normal use)
Second partiton (i.e. “disk0s2): MacOSX - Mac OS Extended (journaled) format (HFS+)
Third partition (i.e. “disk0s3): Shared Data - FAT32 format (up to 8 TB!)
Fourth partition (i.e. “disk0s4): Windows XP - NTFS format
Once in the Windows Setup partition overview, you will notice that the Mac OS Disk Utility creates unallocated spaces of about 128 MB between the partitions. Don’t ask me why, and I wouldn’t recommend changing that with “Partition Magic”, “fdisk”, “GParted” or the like. Chances are that the booting into Windows XP will screw up (Mac OS however remains unaffected, as long you don’t tamper with the Mac OS partition).
The third partition is used to store files that you want to share between the Windows XP and Mac OS, as both can read and write to the third partition (FAT32). However, the max file size is 4,3 GB, which makes things difficult when it comes to disk images, videos, etc.. If this is important to you, you may stop reading here and read the tutorials on how to use NTFS or Linux ext2 or ext3 for the shared partition and by installing corresponding drivers on both systems (for instance using MacDrive on WinXP and Paragon NTFS on Mac).
Both Mac OS and Windows XP need about 10 GB for the operating system and basic office applications, I would recommend 20 or even 30 GB each just to be sure. You definitively need more if you install games or audio/video editing applications. Once formatted, changing the size of the FAT32 or NTFS partitions is not possible anymore without starting from scratch. And believe me, Partition Magic and GParted (until now) do not understand the Mac OS GUID partition table, which needs to be changed in parallel with the MBR table changes these programs modify.
With a little trick, which I’ll tell you in a moment, it is possible to make the FAT32 partition as large as you want (override the 32 GB limit of Windows Setup) and to avoid Windows setup files being copied to your shared Data partition, as it is the first partition Windows Setup “sees”.
Apple’s Boot Camp application CAN NOT be used for this partitioning scheme, because it will only allow to make two partitions. All necessary partition work (except for the NTFS formatting during Windows Setup) is done using Disk Utility in the Mac OS environment or from the Mac OS Install DVD. No other partition application is necessary using this Guideline! And BTW, you don’t need to manually edit the Windows boot file “boot.ini” like other tutorials require out there!
I assume you have a working Mac OS when you start, otherwise you’ll start off the Install DVD installing Mac OS. In that case it’s advisable to format the partitions according to steps 2-4 by using Disk Utility found in the Mac menu before you press “continue” to install OS X (after the initial language choice screen).
Here is how it goes:
1) Backup your data before you begin. For the Mac OS partition this is a measure just to be on the safe side. Either to an USB-Stick, a CD/DVD or to an external hard disk. A simple way of doing that is to use the Mac OS Disk Utility (found in “Utilities” folder in Finders “Go” menu) to clone your Mac data (the whole partition) to an external USB or FireWire drive formatted with HFS+ (or FAT32, but then you have a 4,3 GB file size limit which may be a problem, even if you use compression). In case you also need to backup your current Windows XP partition (this partition and all data along with it will be erased using this guideline), use for instance the free Mac OS application “Winclone” and clone onto a HFS+ partition. Cloning in this case basically means an instant image of the complete partition is written to a large file. Using the restore command means to “put the partition back again”, however overwriting existing and perhaps newer data.
2) Use the Disk Utility, mark your hard disk and go to the “partition” menu. Drag the lower right corner to resize your Mac OS partition to accommodate your intended Data and Windows partition sizes. It will do so without destroying or reformatting your Mac OS partition. Use the “-“ (minus) command or “Erase” menu to free up the space after the Mac OS partition.
3) Now create two new partitions using the “+” (plus) in the partition menu. Choose size and format the third partition to “MS-DOS (FAT)”, which means FAT32. Give it a name like “WinXP”, as this is the partition for Windows XP.
4) Now the trick: Format the second partition using Mac OS Extended (Journaled), not FAT32 at this stage, and call it something like “Data”. This will become your shared Data partition. Use any size you want.
5) Put in your Windows XP CD and go to “System Preferences” and choose the extension “Startup Disk“. Wait until the CD icon appears and choose the Windows Setup CD to boot from.
6) Reboot your Mac into the Windows installer. Sometimes it won’t, in that case hold the "C" key (to boot from CD) or hold the Option key during boot in order to get a list of bootable devices. Select the CD icon.
7) Windows Setup will eventually provide you with an option to format the drive. Be sure to select the correct partition and format with NTFS (you can select fast format unless you are experiencing problems). Just to make it clear: You MUST format here, even if you choose to keep FAT32.
8) Once finished installing Windows XP and the Boot Camp XP Drivers from the Mac OS Install DVD, reboot into Mac OS by pressing Option when the chime sounds and selecting Mac OS (or by choosing Mac OS from the Boot Camp control panel in the Windows system tray).
9) As a last step enter Disk Utility again and chose “Erase” menu. Mark the Data partition and “erase” it as “MS-DOS (FAT)”. This will format the partition in a way fully accessible to both Windows and Mac OS. You'll notice that Disk Utility has no problem converting partitions larger than 32 GB. You’re done!
PS. You can now install “rEIFt”, which is a nice graphical boot loader for multiple OS Macs. During Mac startup, rEFIt will show all bootable options available. Don’t install rEFIt until you know all your OS’s work, because it doesn't provide support for installation of any operating system. However it has the capability to show the GUID Partition Table (GPT) and the MBR table and echo the MBR entries so it fits the GPT (not the other way around). I’ve experienced that rEFIt sometimes needs a second reboot before it is recognized properly.
PS2. Mac OS doesn’t grow and slows down like Windows XP (due to the “registry” and orphaned files), so you can safely use the same partition for your data and use Time Machine to backup to a second drive. For Windows XP I recommend to use MS Tweak UI to change your document, desktop, picture, etc. folder to point at and keep all personal data in your Data partition. You can then continue use Winclone to restore your Windows clone/image to restore Windows to virgin state, for instance if you’ve caught a trojan or a virus.
Guideline Background
Both Mac OS X and Windows can read and write to partitions formatted with FAT and FAT32. Unfortunately, Mac OS X can read but not write to NTFS formatted partitions that Windows NT, 2000, and XP can use instead of FAT32. NTFS should be used for a Windows partition, as it is safer.
A limitation when it comes to partitions is that the Windows MBR can address 4 primary partitions only. The first 2 partitions on Intel Macs are reserved for something called EFI (200 MB, normally invisible) and for the Mac OS itself. That leaves room for 2 additional partitions, one for your shared “bilingual” data and one for Windows XP.
For a strange reason, Windows XP MUST be installed on the LAST partition in order for the EFI boot loader to recognize it (“hal.dll corrupt or missing” or “missing media” error).
So the recommended solution is:
First Partition (i.e. “disk0s1”): reserved EFI partition (invisible in Disk Utility and during normal use)
Second partiton (i.e. “disk0s2): MacOSX - Mac OS Extended (journaled) format (HFS+)
Third partition (i.e. “disk0s3): Shared Data - FAT32 format (up to 8 TB!)
Fourth partition (i.e. “disk0s4): Windows XP - NTFS format
Once in the Windows Setup partition overview, you will notice that the Mac OS Disk Utility creates unallocated spaces of about 128 MB between the partitions. Don’t ask me why, and I wouldn’t recommend changing that with “Partition Magic”, “fdisk”, “GParted” or the like. Chances are that the booting into Windows XP will screw up (Mac OS however remains unaffected, as long you don’t tamper with the Mac OS partition).
The third partition is used to store files that you want to share between the Windows XP and Mac OS, as both can read and write to the third partition (FAT32). However, the max file size is 4,3 GB, which makes things difficult when it comes to disk images, videos, etc.. If this is important to you, you may stop reading here and read the tutorials on how to use NTFS or Linux ext2 or ext3 for the shared partition and by installing corresponding drivers on both systems (for instance using MacDrive on WinXP and Paragon NTFS on Mac).
Both Mac OS and Windows XP need about 10 GB for the operating system and basic office applications, I would recommend 20 or even 30 GB each just to be sure. You definitively need more if you install games or audio/video editing applications. Once formatted, changing the size of the FAT32 or NTFS partitions is not possible anymore without starting from scratch. And believe me, Partition Magic and GParted (until now) do not understand the Mac OS GUID partition table, which needs to be changed in parallel with the MBR table changes these programs modify.
With a little trick, which I’ll tell you in a moment, it is possible to make the FAT32 partition as large as you want (override the 32 GB limit of Windows Setup) and to avoid Windows setup files being copied to your shared Data partition, as it is the first partition Windows Setup “sees”.
Apple’s Boot Camp application CAN NOT be used for this partitioning scheme, because it will only allow to make two partitions. All necessary partition work (except for the NTFS formatting during Windows Setup) is done using Disk Utility in the Mac OS environment or from the Mac OS Install DVD. No other partition application is necessary using this Guideline! And BTW, you don’t need to manually edit the Windows boot file “boot.ini” like other tutorials require out there!
I assume you have a working Mac OS when you start, otherwise you’ll start off the Install DVD installing Mac OS. In that case it’s advisable to format the partitions according to steps 2-4 by using Disk Utility found in the Mac menu before you press “continue” to install OS X (after the initial language choice screen).
Here is how it goes:
1) Backup your data before you begin. For the Mac OS partition this is a measure just to be on the safe side. Either to an USB-Stick, a CD/DVD or to an external hard disk. A simple way of doing that is to use the Mac OS Disk Utility (found in “Utilities” folder in Finders “Go” menu) to clone your Mac data (the whole partition) to an external USB or FireWire drive formatted with HFS+ (or FAT32, but then you have a 4,3 GB file size limit which may be a problem, even if you use compression). In case you also need to backup your current Windows XP partition (this partition and all data along with it will be erased using this guideline), use for instance the free Mac OS application “Winclone” and clone onto a HFS+ partition. Cloning in this case basically means an instant image of the complete partition is written to a large file. Using the restore command means to “put the partition back again”, however overwriting existing and perhaps newer data.
2) Use the Disk Utility, mark your hard disk and go to the “partition” menu. Drag the lower right corner to resize your Mac OS partition to accommodate your intended Data and Windows partition sizes. It will do so without destroying or reformatting your Mac OS partition. Use the “-“ (minus) command or “Erase” menu to free up the space after the Mac OS partition.
3) Now create two new partitions using the “+” (plus) in the partition menu. Choose size and format the third partition to “MS-DOS (FAT)”, which means FAT32. Give it a name like “WinXP”, as this is the partition for Windows XP.
4) Now the trick: Format the second partition using Mac OS Extended (Journaled), not FAT32 at this stage, and call it something like “Data”. This will become your shared Data partition. Use any size you want.
5) Put in your Windows XP CD and go to “System Preferences” and choose the extension “Startup Disk“. Wait until the CD icon appears and choose the Windows Setup CD to boot from.
6) Reboot your Mac into the Windows installer. Sometimes it won’t, in that case hold the "C" key (to boot from CD) or hold the Option key during boot in order to get a list of bootable devices. Select the CD icon.
7) Windows Setup will eventually provide you with an option to format the drive. Be sure to select the correct partition and format with NTFS (you can select fast format unless you are experiencing problems). Just to make it clear: You MUST format here, even if you choose to keep FAT32.
8) Once finished installing Windows XP and the Boot Camp XP Drivers from the Mac OS Install DVD, reboot into Mac OS by pressing Option when the chime sounds and selecting Mac OS (or by choosing Mac OS from the Boot Camp control panel in the Windows system tray).
9) As a last step enter Disk Utility again and chose “Erase” menu. Mark the Data partition and “erase” it as “MS-DOS (FAT)”. This will format the partition in a way fully accessible to both Windows and Mac OS. You'll notice that Disk Utility has no problem converting partitions larger than 32 GB. You’re done!
PS. You can now install “rEIFt”, which is a nice graphical boot loader for multiple OS Macs. During Mac startup, rEFIt will show all bootable options available. Don’t install rEFIt until you know all your OS’s work, because it doesn't provide support for installation of any operating system. However it has the capability to show the GUID Partition Table (GPT) and the MBR table and echo the MBR entries so it fits the GPT (not the other way around). I’ve experienced that rEFIt sometimes needs a second reboot before it is recognized properly.
PS2. Mac OS doesn’t grow and slows down like Windows XP (due to the “registry” and orphaned files), so you can safely use the same partition for your data and use Time Machine to backup to a second drive. For Windows XP I recommend to use MS Tweak UI to change your document, desktop, picture, etc. folder to point at and keep all personal data in your Data partition. You can then continue use Winclone to restore your Windows clone/image to restore Windows to virgin state, for instance if you’ve caught a trojan or a virus.
I found it extremely useful! In particular, the bits about GUID, MBR, 128MB spacing partitions and external partitioning software were most helpful. Cheers, Sean 
