I have a 500 GB external drive that I use with my Windows 7 PC and Snow Leopard laptop. It has the following 4 partitions:

 1. Leopard Installer (HFS+)
 2. Backup (HFS+)
 3. Snow Leopard Installer (HFS+)
 4. Storage (NTFS)

1 and 3 contain Mac OS X install CD images in case of any problems, and 2 contains a bootable backup of my entire Mac. The last partition is the one I store movies, music and docs on. The PC recognizes only partition 4, while the Mac recognizes all 4, which is perfect. All this is with the GUID Partition Table (GPT). But since its recognized on both Macs and PCs, I'm guessing it uses something like a hybrid MBR. I don't know what that means, but I remember having to do a lot adjustment to get it to play well with both my computers.

However, I no longer needed the leopard installer, so I erased that partition using disk utility, and formatted it with exFAT, and then again with normal FAT, so that I could use it with my fonera. Now, the Windows PC recognizes the FAT partition, but not the NTFS one it used to. It shows up in Disk Management as Unallocated space, but EASEUS partition manager can read the files off it, and the Mac recognizes it fine.

Any ideas about what's wrong or how I can fix it?

Update (diskpart.exe output):

Microsoft DiskPart version 6.1.7601

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online           37 GB  2048 KB        *
  Disk 1    Online          149 GB      0 B
  Disk 2    Online          465 GB      0 B

DISKPART> select disk=2

Disk 2 is now the selected disk.

DISKPART> list partition

  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    Primary            200 MB    512 B
  Partition 2    Primary             10 GB   201 MB
  Partition 3    Primary            116 GB    11 GB
  Partition 4    Primary             17 GB   128 GB

It's not listing the 300 or so GB Storage partition.

  • Are these all 'primary' partitions? Which partition(s) are set as 'active'? – Ƭᴇcʜιᴇ007 Jun 16 '11 at 16:28
  • @techie007, I just checked Disk Management, all of them are marked as Primary partition except the NTFS one. Is that the key to it being given a drive letter? – Neil Jun 16 '11 at 16:34
  • 2
    Run diskpart and run the list disk command. It will tell you what partitioning scheme is used. Then run the list partition command, which will tell you exactly what Windows is seeing as your partition table. Place that information in your question. – JdeBP Jun 16 '11 at 16:57
  • Sorry, I ran that before, but didn't think uploading it here would help. The storage partition refuses to show up on this, but it did show up in EASEUS Partition Manager, should I add a link to a screenshot, or any data in particular? – Neil Jun 16 '11 at 18:21
  • Uploading it here most definitely does help. With the output of diskpart it's fairly clear what's going on. However before I write an answer, run detail partition on each of those four partitions on that disc and add that information. The output will probably tell you straightaway what is happening. It almost certainly will tell one or more people reading this. – JdeBP Jun 16 '11 at 21:25

You changed the partition table when you erased the second partition, and confused the heck out of Windows. Your easiest (!) way to fix this is to move the files off the NTFS partition, recreate it, and move the files back.

You must do this in Windows 7.

For future reference, I'd put the NTFS partition first or second on the drive. I think there are partition boundaries you are playing with that caused the confusion (and changing the disk order of recognisable partitions in Windows), despite Microsoft's promises otherwise, but I may be wrong.

My reason for stating this is as follows: I have a Mac with one hard drive, partitioned into three drives. First is the main OS X partition, second is a second OS X partition for testing (I'm under NDA so I can't tell you what's there), and third is my Windows 7 under Boot Camp. When I created the second OS X partition for testing, Disk Utility warned me that Boot Camp may no longer work because I was changing the partition table.

  • I had a hunch that Windows might be assigning a drive letter to only the first partition it can read on the disk. Earlier, the first three partitions were all HFS, so it would give me the last, NTFS partition. Now that it can read the FAT partition, it shows me that, but not the NTFS one. Could that be possible? – Neil Jun 18 '11 at 7:45
  • Yes, it's possible. – user3463 Jun 18 '11 at 18:23

The problem is almost certainly that the disk uses a hybrid MBR, which is a standards-violating hack that Apple uses to enable Windows to boot on their computers. A hybrid MBR disk is a GPT disk with up to three partitions duplicated on the MBR side (plus a special MBR partition that identifies the disk as a GPT disk). Windows sees the MBR side of such a disk and OS X sees the GPT side, so you can see all the partitions in OS X but no more than three from Windows. Which three depends on which partitions were selected for inclusion in the MBR by whatever tool set up the hybrid MBR.

Since this is an external disk with no bootable OS on it, it doesn't need the hybrid MBR configuration; either a conventional MBR or a conventional GPT will do the trick. If the disk is only accessed from OS X and Windows 7, my inclination would be to go for a straight GPT setup, since that's more flexible if and when you need to add more partitions; however, OS X's Disk Utility might decide to convert it back to a hybrid MBR if you use it to create or format FAT or NTFS volumes. Also, if you intend to use the disk with certain (mostly older) OSes, such as Windows XP, MBR is a better choice, since these OSes don't understand GPT.

To rid yourself of the hybrid MBR, you'll need to create a fresh "protective MBR," which is the standard part of a GPT disk that the hybrid MBR replaces. Some partitioning tools, such as Linux's GParted, will do this automatically if you use them to make changes to the disk; but since you didn't mention Linux, I'll assume you don't have ready access to GParted. You could instead try GPT fdisk (gdisk), which is available for Linux, OS X, and Windows. (Running it under OS X is a bit safer because if you launch gdisk on the wrong disk and then save your changes under Windows, you'll convert your Windows boot disk to GPT form, which would be Bad with a capital B. The risk of doing this is low, though. OS X uses GPT natively, so this isn't an issue for OS X.) You should do this:

  1. Install the gdisk package for your platform.
  2. Launch gdisk on your disk. In Windows, you'd open an Administrator Command Prompt and type "gdisk 1:" (assuming this is the second disk, the first being your internal disk). In OS X, you'd open a Terminal window and type "sudo gdisk /dev/disk1" (again, assuming it's the second disk).
  3. If you're using the Windows version, it'll beep and warn you that Windows can't boot from GPT disks on most computers. Go on and enter the program; you're using it on a non-boot disk.
  4. Type "p" to view your partition table and verify you're working on the correct disk. If not, type "q" and try again with a different disk number.
  5. Type "x" to enter the experts' menu.
  6. Type "n" to create a new protective MBR. The program won't display any messages; it'll just show you a new command prompt.
  7. Type "w" to save your changes. You'll be asked for confirmation. Give it.

That's it. You should now be able to see all your partitions from both OSes, except of course that you probably lack HFS+ drivers in Windows, so those partitions won't yield drive letters. (They will show up in the Windows disk partitioning software, though.)

If you want to convert the disk to a straight-up MBR form, you can do that with gdisk, too; but you'd type "r" at the main menu to enter the recovery & transformation menu, followed by "g" to convert from GPT to MBR, followed by "w" to save your changes.

  • Wow, thanks for that really comprehensive answer. I feel really bad saying this, but I took the nuclear option that all Windows users have gotten used to by now - wipe the slate clean and start fresh. I did have a bootable OS on the external drive, for the bootable ghost backup of my Mac's internal drive, but I've decided to get rid of it. Now I'm copying the contents of the disk using a mac, formatting the entire disk to one non-journaled HFS+ partition, and installing MacDrive on Windows 7, so that the drive can be accessed from Mac, PC and Fonera. – Neil Jun 23 '11 at 9:27

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