A quirk in the Apple Disk Utility allowed me to create a FAT32 volume that would not mount in Windows. Avoiding the same problem is easy once you have an understanding of how Apple’s formatting tool works. Read on, support pros, and save yourselves from future headaches.


I support both Macs and Windows machines, so I spend a lot of time running between systems. There are a few features in the MacOS that I rely on to make my double life a little easier, one of those being the built-in support Apple computers have for Windows volumes.

Since Macs can read and write to FAT32 partitions,* I can keep software and utilities for both operating systems on a single drive. That feature helps keep my tool kit a little lighter and my life a little simpler. In fact, when it comes to working with FAT32 partitions, Mac OS can do something natively that Windows cannot: MacOS can format FAT32 partitions larger than 32 GB. I frequently use MacOS to format FAT32 volumes, and that feature has historically worked for me without a hitch. At least, it did until last week, when I discovered a quirk in the MacOS Disk Utility that allowed me to create a FAT32 volume that would not mount on Windows.

* Microsoft recommends their NTFS format for large Windows-compatible disks. Apple machines can natively read NTFS volumes, but they can’t write to that file system without third-party software. Look into the Mac port of the NTFS-3G project if you need to write to NTFS disks from your Mac.

The Disk Utility in MacOS version 10.5 is the nexus of file system management on your Apple computer. From that tool you can create partitions and disk images, format volumes, and mount files systems. Disk Utility was the obvious destination for me when I decided to format an old USB drive I had lying around. I needed to copy some files from my PlayStation 3, and I thought I would reformat an external HD for that purpose. The USB drive had some old Mac backups on it and was set up in the Apple-recommended HFS+ format. The PS3 will mount only FAT32 drives, so I thought I’d erase my drive and quickly reformat the volume. Then I’d be off to the races. Not so fast…

When I took my drive over to my PS3, I discovered it wouldn’t mount. The drive wouldn’t mount on my Windows machine, either. Only after investigation did I discover that I had greatly misunderstood how I should have been using Apple’s Disk Utility.

Disk Utility displays a number of action palettes when you select a storage device from its left-hand pane. Two of these palettes, “Erase” and “Partition,” appear to accomplish the same thing, but there is a very significant difference. The Erase pane does not allow you to change your drive’s partition map.

When I used the Erase tool on my HFS+ formatted drive, which carried an Apple Partition Map, Disk Utility obediently erased the HFS+ file system and replaced it with the FAT32 file system I asked for. It did not, however, create a Master Boot Record to replace the Apple Partition Map on the disk. So, when I tried to mount the disk on my PC and my PS3, both of which are devices that require an MBR to get their file system information, I was out of luck.

Editing a partition table is a separate administrative task in Mac OS, distinct from merely erasing a volume, which is probably pretty smart. I ran into this problem because I was jumping between formatting schemes on my disk. If I had not wanted to change the disk format my drive was using, I wouldn’t have encountered this issue. But then I would have missed out on a valuable learning experience, right?

If you want to make sure that the FAT32 disks you create with your Mac are completely Windows compatible, follow these steps:

Once your disk is selected, make sure to switch to the Partition palette, which is highlighted in Figure A.
Figure A

On the Partition palette, follow the numbers.

Step 1: Use the Volume Scheme pull-down menu to chose the number of partitions you want (Figure B).
Step 2: Go to the Options… pane and make sure Master Boot Record is selected (Figure C).

Step 3: Choose MS-DOS from the Format pull-down menu. Click Apply to reformat your drive.

Figure B

The Options pane in the Partition tool is very clear about what partition schemes can be used where.

Figure C

Using FAT32 volumes is a good way to share files across platforms if you use both MacOS and Windows. Apple’s Disk Utility is a powerful tool for setting up such disks, but being ignorant of how to use its advanced options caused me a little trouble. Now that I have an understanding of the process, I won’t make the same mistake twice.