Will Mountain Lion destroy 3rd data partition?

Discussion in 'Windows, Linux & Others on the Mac' started by vistadude, Aug 18, 2012.

  1. vistadude macrumors 65816

    Joined:
    Jan 3, 2010
    #1
    I have a macbook with Snow Leopard, Windows 7 via bootcamp, and a 3rd FAT32 partition for saving data. These are all from a single internal hard disk.

    I read that mountain lion creates an additional hidden partition to save the OS. Will this screw up my already existing FAT32 partition or bootcamp?

    Thanks in advance.
     
  2. vistadude thread starter macrumors 65816

    Joined:
    Jan 3, 2010
    #2
    Bump,

    I'm giving this laptop to my parents who are more than 60 years old. I'm leaving town tomorrow night, if someone could provide help, I'd appreciate it.

    My parents don't ever update their computers and they always get viruses. I'm hoping to get them the latest version of OS X and Windows 7 so next time I visit, their credit card numbers aren't stolen :)
     
  3. murphychris macrumors 6502a

    Joined:
    Mar 19, 2012
    #3
    You haven't provided enough information. But based on what you've provided, my tentative answer is, upgrading to Mountain Lion will render Windows unbootable.

    Mac OS depends on GPT. Windows on Apple hardware depends on MBR. MBR only supports four partitions. In a "normal" situation with Boot Camp there are already four partitions: EFI System, Macintosh HD, Recovery HD, Windows. There is no more room for a fifth "FAT32" partition to fit into the MBR. Upon seeing this fifth partition, Apple's tools appear to wipe out the hybrid MBR, and replace it with a PMBR (single entry MBR, making the disk GPT only), and that renders Windows unbootable.

    If you want to do yourself and your parents a favor, put Windows 7 in a VM instead of using Boot Camp. It's a huge PITA, especially if anything goes wrong. It literally invites data loss in my opinion.

    ----------

    To provide more information for a less tentative answer, please post the results for the following Terminal command. Please highlight the pasted text, and click the # button in the forum toolbar to properly format it.

    Code:
    sudo gpt -r -vv show disk0
     
  4. vistadude thread starter macrumors 65816

    Joined:
    Jan 3, 2010
    #4
    Thanks for the quick response. The problem is dad wants windows, mom wants mac. They won't use a virtual machine. Here is what I got from the command you showed.

    gpt show: disk0: mediasize=250059350016; sectorsize=512; blocks=488397168
    gpt show: disk0: Suspicious MBR at sector 0
    gpt show: disk0: Pri GPT at sector 1
    gpt show: disk0: Sec GPT at sector 488397167
    start size index contents
    0 1 MBR
    1 1 Pri GPT header
    2 32 Pri GPT table
    34 6
    40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
    409640 136456608 2 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
    136866248 263736
    137129984 286906368 3 GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
    424036352 64360448 4 GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
    488396800 335
    488397135 32 Sec GPT table
    488397167 1 Sec GPT header
    macbook-pc:~ user1$
     
  5. murphychris macrumors 6502a

    Joined:
    Mar 19, 2012
    #5
    OK so not to be a dick, but what part of Please highlight the pasted text, and click the # button in the forum toolbar to properly format it. wasn't understandable?

    ----------

    The GPT already contains 4 partitions. I can't tell you for sure if Mountain Lion will successfully wedge in a new partition for Recovery HD (Lion tended to fail to do this frequently). If it's successful the Windows partition will not be bootable. You'll need to use gdisk to create a new hybrid MBR, where you will add the last two partitions to the MBR (you'll need to check the GPT again after the upgrade to see what the partition numbers are).

    In effect this stuffs the GPT, EFI System, and Macintosh HD into MBR partition 1 as type 0xEE (GPT protected partition) and thus those partitions will be ignored by Windows. It will only see the last two which are "Windows basic data" partitions. Mac OS X will continue to see all of the partitions since it uses GPT and ignores the MBR.

    Again, I advise using VM for Windows.
     
  6. vistadude thread starter macrumors 65816

    Joined:
    Jan 3, 2010
    #6
    I pressed it the pound button, but it didn't work. Maybe it's an opera thing, I'm trying Safari this time.

    Code:
    gpt show: disk0: mediasize=250059350016; sectorsize=512; blocks=488397168
    gpt show: disk0: Suspicious MBR at sector 0
    gpt show: disk0: Pri GPT at sector 1
    gpt show: disk0: Sec GPT at sector 488397167
          start       size  index  contents
              0          1         MBR
              1          1         Pri GPT header
              2         32         Pri GPT table
             34          6         
             40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
         409640  136456608      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
      136866248     263736         
      137129984  286906368      3  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      424036352   64360448      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      488396800        335         
      488397135         32         Sec GPT table
      488397167          1         Sec GPT header
    macbook-pc:~ user1$ 
    
     
  7. murphychris macrumors 6502a

    Joined:
    Mar 19, 2012
    #7
    Yeah, I was able to figure it out, so the same answers as earlier apply. The MBR can't hold 5 partitions, and currently Apple's tools fail to create a hybrid MBR when the GPT contains 5+ partitions, and without a hybrid MBR Windows will not boot on Apple hardware.
     
  8. vistadude thread starter macrumors 65816

    Joined:
    Jan 3, 2010
    #8
    Thanks for your help! I guess i'm stuck with snow leopard and windows 7 unless i want to use a virtual machine.
     
  9. murphychris macrumors 6502a

    Joined:
    Mar 19, 2012
    #9
    Or go down the rabbit hole of using gdisk to create a new hybrid MBR after you've done the Mountain Lion upgrade. Instead of adding every GPT partition to a corresponding MBR partition (which you can't do since you'll have 5 GPT partitions and MBR can only hold 4); you can add just the last two GPT partitions (FAT32 and Windows 7) to the MBR. The earlier GPT partitions will all be defined in the MBRs first partition (one MBR partition for three GPT partitions) marked as the GPT protected partition.

    I advise VM. I advise VM. I advise VM.

    With VM, your disk management is VASTLY simpler, less fragile, disk space consumption is far less yet still growable without having to worry about resizing later, and it's easier to backup and restore down the road should they outgrow this drive.
     
  10. jrdempster macrumors newbie

    Joined:
    Jul 19, 2012
    #10
    My setup was very similar and I upgraded with no problem.

    Originally, 3 usable partitions:

    1. OS X, Snow Leopard
    2. Fat32, Shared data partition
    3. Win 7

    The upgrade to Lion preserved all 3, did not attempt to create a recovery partition.

    The upgrade to Mountain Lion, same thing. All 3 partitions preserved intact, just the OS X partition upgraded.

    You can separately create the recovery partition on an external HD.

    Of course, backup all partitions first, in case your experience is different.
     

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