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I have an external HDD that I formated in FAT32 using Mac OS X. I don't know how but it's unmountable in a Windows XP machine running SP3. The HDD size is 500Gb and it contains a single partition. Is there a way to make it mountable on a Windows machine without formatting it again?

I've checked the partition map scheme with Disk Utility and it's set to Apple Partition Map.

[edit]I would prefer a free tool but I don't mind learning about paid tools.[/edit]

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Which version of Windows was this? I know there are issues before XP SP1 with drives over a specific size, regardless of which partition type it is. – Powerlord Sep 30 '09 at 15:41
It's Windows XP SP3. I've edited the question to include that detail. – Nuno Curado Sep 30 '09 at 15:43
I haven't forgotten to accept one of the answers. I just had some really busy days. Hopefully I'll be able to test one last answer by the weekend and then I'll accept one. – Nuno Curado Oct 8 '09 at 9:32
up vote 2 down vote accepted

when you create FAT32 partitions with a GUID partition scheme, XP cannot read the drive, requires MBR.

on a Mac click on the disk in disk utility and see what it shows under "Partition Map Scheme".

it is not possible to change from GUID to MBR without setting up the drive again, on a MAC with the correct option or on a Windows computer via disk management. either way, all data on the drive will be overwritten.

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How can I change to MBR? – Nuno Curado Sep 30 '09 at 15:56
Thanks. I'll check it later today. – Nuno Curado Sep 30 '09 at 16:00
probably not without formatting again. but why using FAT32? use NTFS, much faster than FAT32 and not capped a 4GB file size. you can read/write NTFS disks on a Mac.… – Molly7244 Sep 30 '09 at 16:08
I used NTFS before but somehow it corrupted some of my mp3 files and I decided to go back to FAT32. – Nuno Curado Sep 30 '09 at 21:03
Molly - MacFUSE is still tagged experimental. I wouldn't recommend anyone use it for anything important. – MDMarra Sep 30 '09 at 21:39

When you format a drive under OSX, it uses the EFI partition table format (also called GUID), which isn't compatible with Windows (XP, Vista, or 7).

To resolve this, you need to format the drive as FAT32, and when you do so, click the small "Options" button in Disk Utility. You'll see this:

Disk Utility Formatting Options

You need to select "Master Boot Record" for the drive to work correctly under Windows. It's probably worth noting that there are more than likely a few ways mentioned in other answers to get the disk working without reformatting it.

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Thanks for the reply but it doesn't answer my question. I now know how to prevent the issue whenever I format a new drive but I still don't have an answer on how to solve it without re-formatting. – Nuno Curado Oct 1 '09 at 8:39
I mentioned this towards the ending, however it may have been a little cryptic. You cannot fix this issue without reformatting the thumb drive unless you are willing to use one of the third party tools mentioned in other answers. – EvilChookie Oct 1 '09 at 8:58

iPartition from Coriolis can change the Partition Map Scheme from Apple Partition Map to MBR. At the time of this answer there were no free tools available.

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You need a utility to mount it, such as MacDisk.

enter image description here

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I'll have to wait until the weekend to test this answer. – Nuno Curado Sep 30 '09 at 21:05

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