Edit Master Boot Record partition scheme map

Discussion in 'Mac Basics and Help' started by rufus247, Mar 1, 2010.

  1. rufus247 macrumors member

    Jan 26, 2009
    I am having problems. I want to delete a partition and create a new one, but I need to be able to click option in able to do this. For whatever reason, disk utility will not let me edit my external in any way without deleting the whole thing which I do not want to do. I need help. I have posted a picture.

    Attached Files:

  2. spinnerlys Guest


    Sep 7, 2008
    forlod bygningen
    That is most likely due to the Partition Map Scheme (bottom right of your Disk Utility window) that is set for the external HDD.

    It is set to Master Boot Record, which is used for Windows computers and Mac OS X can't seem to handle resizing with those.
  3. rufus247 thread starter macrumors member

    Jan 26, 2009
    But i did all reformatting on my mac. Is there anything I can do?
  4. spinnerlys Guest


    Sep 7, 2008
    forlod bygningen
    Not that I know of besides backing up your data and reformatting the HDD again and making sure you selected GUID as partition table/map via the Options button.


    Maybe change your thread title to something like: "How to resize partitions on an HDD using the Master Boot Record partition scheme map" or something shorter.

    To edit your thread title, just click on the [​IMG] button on the bottom right of your original post and then click the "Go Advanced" button below your message.
    Have you also taken a look at MRoogle, since that question may have been asked several times?
  5. rufus247 thread starter macrumors member

    Jan 26, 2009
    Thanks. I do not want to have to copy all of my data again.
  6. bobr1952 macrumors 68020


    Jan 21, 2008
    Melbourne, FL
    This was something that caught me as well. Another item for Windows switchers to look out for--and this one I have to say I had no clue about. I just knew that my iMac was not recognizing my "bootable" external drive as being bootable--until I discovered the GUID Partition Table.
  7. aarons510 macrumors newbie

    Jun 4, 2010
    Using fdisk to resize last partition on an MBR disk.

    Actually, one can use the command-line utility fdisk to resize the last partition on an MBR-partitioned disk that has at most four partitions, provided you back up and restore whatever contents of the last partition you want to preserve.

    The most straightforward thing to do is:

    1) Do a Get Info in Disk Utility on the drive you want to modify to obtain its Disk Identifier, which will probably be in the form diskn, where n is a small single digit.

    2) Also using Disk Utility, back up the volume (partition) you are going to modify (if it has anything you want to save on it) by creating an image file of said volume. If there are any other files or volumes on the disk that you really, really don't want to risk losing, back those up also, just in case!

    3) Open Terminal and type

    fdisk -e /dev/<disk identifier>
    where '<disk identifier>' is the name you obtained in step 1. Ignore the message:

    fdisk: could not open MBR file [...]
    that you will probably then see.

    4) Type
    p <RETURN>
    to see a list of partitions on the disk. Then type
    edit <n>
    where <n> is the number (from 1 to 4) of the partition you want to edit, which had better be the last one on the list of partitions unless you are knowledgeable enough not to have needed these instructions in the first place. You will be given a sequence of prompts that you can respond to by pressing <RETURN> to choose the default. However, if you don't want to use up all the space at the end of the disk for the partition you are expanding, you can respond to the 'Partition size' prompt with a smaller number. In addition, if the default response to the 'Partition offset' prompt is a lower number than the existing start sector number for that partition, you may want to respond to the prompt with the latter number, so that the starting location of the volume remains unchanged.

    5) After you do this, type 'p' again to make sure that you haven't changed any other partitions, then type 'w' to write the new partition table to the disk. Despite any warnings you will see before confirming this, writing the edited partition table to disk will not change anything except the partition (volume) whose entry you edited, and, in most cases, won't change any data even on the edited volume, although the OS may, when that volume is mounted, treat it as unreadable or problematic. In that case, you may want to restore the contents of that volume, or some part of them, from the backup you made in step 2.
  8. Sottilde macrumors newbie

    Mar 30, 2012
    Thank you, aarons510! I had a 1TB NTFS external that I wanted to carve a 150GB HFS+ partition out of to use as a Time Machine backup for my Air. I resized the existing partition using compmgmt.msc in Windows, but when I hooked it up to my Air I wasn't able to create a partition out of the empty space.

    Fdisk did it just fine. I used partition #2 as it was the first unused/empty (00) partition.

    Only little hangup I found was the prompt on whether or not to use CHS mode. Default is 'n', I used that. Literature I could find on the subject suggested that CHS mode involves sizing the disk with Cylinders, Heads, and Sectors, which are often bogus on newer drives. You don't need it anyhow.

    After creating the HFS+ partition (AF), I rebooted the machine and started up Disk Utility. I couldn't get the HFS+ partition to mount until I formatted it within Disk Utility - thankfully that only takes about 10 seconds.

    Anyway, the backup is running happily now - thanks!

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