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Thread: 8.04 cleared the MBR partition table

  1. #21
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    Question Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by pxwpxw View Post
    1. You will still need grub to boot ubuntu.

    2. I have done some tests for that situation - clearing the mbr sector and recovering using an external system. It is described in the link below, but you need to use the ubuntu version of the echo command to get the hex55aa. If you have any problem I can make you a 512byte file which will let refit recreate the full MBR as described, after using the dd command from macosx. You need to be very careful.

    http://ubuntuforums.org/showpost.php...17&postcount=3

    Alternately I might be able to give you a copy of my GPT mbr that will be just enough to allow Macosx to boot and then refit to sync for your gpt tables and mbr.
    Probably try that also.
    It is also possible to use a ubuntu CD if it will boot.

    But first of all, get a hexdump of what is on the mbr sector now and check that it is indeed 512 bytes of 00.
    (If you have not already done that).
    I do indeed have 512 bytes of zeroes. I have installed rEFIt on the external drive and when I try using the partition editor it reports no MBR press any key to continue (approximately, that's not precisely what is said) and does not sync GPT to MBR. How would I go about creating said 512 byte file, and where/how would I place the file for rEFIt?

    Thanks,
    Gregory

  2. #22
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    Angry Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by thekid42 View Post
    I'm actually having a very similar problem to applesjgtl's...

    Initially, I wanted to triple boot my macbook pro, but I found that I could no longer boot into XP after I installed Ubuntu. I decided to remove Windows and Ubuntu to try to start fresh. I then tried clearing the MBR to get rid of GRUB (I messed up when installing Ubuntu and put it in the MBR rather than on my Ubuntu partition...), but now my machine doesn't boot.

    I was able to boot up on Leopard from an external drive and run the rEFIt Partition Inspector, which gave me the following:



    Also, when I run "sudo fdisk /dev/rdisk0", I get:



    I tried using the Partitioning Tool that rEFIt provides when I boot up, but it was unable to sync the gpt and mbr. Now I'm trying to figure out a way to manually rebuild the MBR based off of the GPT info I was able to get from the Partition Inspector.

    Any help would be greatly appreciated as I have two CS finals this week, and a functional computer kind of helps for studying. Thanks in advance.
    that's exactly what my problem is...

  3. #23
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by thekid42 View Post
    I'm actually having a very similar problem to applesjgtl's...

    Initially, I wanted to triple boot my macbook pro, but I found that I could no longer boot into XP after I installed Ubuntu. I decided to remove Windows and Ubuntu to try to start fresh. I then tried clearing the MBR to get rid of GRUB (I messed up when installing Ubuntu and put it in the MBR rather than on my Ubuntu partition...), but now my machine doesn't boot.

    I was able to boot up on Leopard from an external drive and run the rEFIt Partition Inspector, which gave me the following:



    Also, when I run "sudo fdisk /dev/rdisk0", I get:



    I tried using the Partitioning Tool that rEFIt provides when I boot up, but it was unable to sync the gpt and mbr. Now I'm trying to figure out a way to manually rebuild the MBR based off of the GPT info I was able to get from the Partition Inspector.

    Any help would be greatly appreciated as I have two CS finals this week, and a functional computer kind of helps for studying. Thanks in advance.
    Please use the external Macosx terminal to do a hexdump of the 512 bytes of the MBR sector. I am doing this example from my external drive wd:~ pxw$
    Code:
    wd:~ pxw$ sudo hexdump -Cn512 /dev/disk1
    it may be /dev/disk0.
    check using
    Code:
    wd:~ pxw$ diskutil list
    post the result.

  4. #24
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by pxwpxw View Post
    Please use the external Macosx terminal to do a hexdump of the 512 bytes of the MBR sector. I am doing this example from my external drive wd:~ pxw$
    Code:
    wd:~ pxw$ sudo hexdump -Cn512 /dev/disk1
    it may be /dev/disk0.
    check using
    Code:
    wd:~ pxw$ diskutil list
    post the result.
    00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
    *
    00000200

    Mostly zeros, other than that 2?

  5. #25
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by applesjgtl View Post
    I do indeed have 512 bytes of zeroes. I have installed rEFIt on the external drive and when I try using the partition editor it reports no MBR press any key to continue (approximately, that's not precisely what is said) and does not sync GPT to MBR. How would I go about creating said 512 byte file, and where/how would I place the file for rEFIt?

    Thanks,
    Gregory
    I can post a 512 byte dummy mbr sector in a file, I have checked this method here. You will have to download this, check it in a macosx terminal with hexdump, and if it is ok write it to the mbr sector using the dd command in a terminal. Then you can probably boot Macosx on the internal drive, or run refit and it will now see that there is an MBR table to synchronise.

    Please say if you understand that.

  6. #26
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by pxwpxw View Post
    Please use the external Macosx terminal to do a hexdump of the 512 bytes of the MBR sector. I am doing this example from my external drive wd:~ pxw$
    Code:
    wd:~ pxw$ sudo hexdump -Cn512 /dev/disk1
    it may be /dev/disk0.
    check using
    Code:
    wd:~ pxw$ diskutil list
    post the result.
    Code:
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    00000200
    Looks like the exact same result as what applesjgtl got...


    EDIT:
    I can post a 512 byte dummy mbr sector in a file, I have checked this method here. You will have to download this, check it in a macosx terminal with hexdump, and if it is ok write it to the mbr sector using the dd command in a terminal. Then you can probably boot Macosx on the internal drive, or run refit and it will now see that there is an MBR table to synchronise.
    I understand everything except checking it with hexdump. What exactly should we see when we do that?

  7. #27
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    Question Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by pxwpxw View Post
    I can post a 512 byte dummy mbr sector in a file, I have checked this method here. You will have to download this, check it in a macosx terminal with hexdump, and if it is ok write it to the mbr sector using the dd command in a terminal. Then you can probably boot Macosx on the internal drive, or run refit and it will now see that there is an MBR table to synchronise.

    Please say if you understand that.
    I understand what I have to do, though I'm not sure exactly what command I would enter with hexdump and dd. I don't really know all that much about writing unix commands.

  8. #28
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by applesjgtl View Post
    I understand what I have to do, though I'm not sure exactly what command I would enter with hexdump and dd. I don't really know all that much about writing unix commands.
    I will post an example and an attached 512byte dummymbr and how to check it and write it. Take 15 minutes or so to get that ready with the attachment.

  9. #29
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    Smile Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by pxwpxw View Post
    I will post an example and an attached 512byte dummymbr and how to check it and write it. Take 15 minutes or so to get that ready with the attachment.
    okay, thanks a million for your help.

  10. #30
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    Re: 8.04 cleared the MBR partition table

    Quote Originally Posted by applesjgtl View Post
    okay, thanks a million for your help.
    Check the attachment like this, you should see exactly this result, or we may need to do it another way.

    I had to rename it with .txt extension to attach it.

    This is from terminal in macosx.
    Code:
    wd:~/test pxw$ hexdump -C dummymbr 
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    000001c0  01 00 ee fe ff ff 01 00  00 00 af ea 42 25 00 00  |............B%..|
    000001d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
    00000200
    I will post next step.
    Attached Files Attached Files

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