I am running Yosemite version 10.10.5. I have a second partition running Windows 10. My main partition, running OS X, is formatted as Mac OS Extended (Journaled). I try to follow the instructions in the error message, but the Option button is not available.

I do not know how to proceed, and would appreciate some help.

enter image description here

Terminal output:

diskutil cs list:

No CoreStorage logical volume groups found

sudo gpt -r show /dev/disk0:

gpt show: error: bogus map
gpt show: unable to open device '/dev/disk0': Undefined error: 0

sudo fdisk /dev/disk0:

Disk: /dev/disk0    geometry: 29185/255/63 [468862128 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
 1: AF   25 127  15 - 1023 254  63 [    409640 -  400094952] HFS+        
 2: AB 1023 254  63 - 1023 254  63 [ 400504592 -    1269536] Darwin Boot 
*3: 07 1023 254  63 - 1023 254  63 [ 401785650 -   67071375] HPFS/QNX/AUX
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused  

sudo gdisk /dev/disk0:

sudo: gdisk: command not found
  • The disk already uses the GUID partition table, otherwise Yosemite wouldn't be booting. Or do you try to install to an Hackintosh? So don't follow the instructions! Or backup all volumes before "changing" it!
    – klanomath
    Nov 9, 2015 at 16:02
  • No, I have an authentic Apple computer. The point is, I do not know what to change. If the disk is already partitioned correctly, why can't I update the OS?
    – Yoda
    Nov 9, 2015 at 17:20
  • @klanomath Done
    – Yoda
    Nov 9, 2015 at 17:32
  • That looks bad. Please also add sudo fdisk /dev/disk0. Consider downloading and installing gdisk. Try sudo gdisk /dev/disk0 and then from within gdisk p and post the printed partition table if available. The Windows partition and the Mac partition are both bootable? Did you add a second Windows partition from within Windows?
    – klanomath
    Nov 9, 2015 at 17:44
  • I had trouble installing bootcamp on my own, so a friend did it. He said it was complicated. I am not sure what he did. Some kind of workaround, I guess. Both partitions are bootable.
    – Yoda
    Nov 9, 2015 at 17:53

2 Answers 2


Considering the comments and the results of the different listings above, Yosemite probably was installed with a hack to enable a Windows MBR only install.

I assume it was the Yosemite MBR Patch 10.10 (14A389). The patch allows you to install Yosemite to a disk with a pure MBR partition table and not the default GUID partition table with a pMBR or hybrid MBR (if Windows is installed).

Depending on the efforts needed to save your data, you may either backup everything, reinstall first Yosemite (or El Capitan) from scratch after partitioning the main disk with a GUID partition table and migrate your data from the backup and then install Windows with Boot Camp Assistant or create a GUID partition table with proper partition values manually after booting to Internet Recovery Mode. The latter will probably render Windows unbootable.

Since the first method is trivial I only describe the second one.

Add a GUID partition table manually:

This method will probably render Windows unbootable!


  • Backup your data - especially any data on your Windows partition.
  • Detach any external drive (especially your external Time Machine backup drive).
  • Restart to Internet Recovery Mode by pressing alt cmd R at startup.
    The prerequisites are the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated.
    On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot to a recovery netboot image which usually is loaded from an apple/akamai server.

    I recommend ethernet because it's more reliable. If you are restricted to WIFI and the boot process fails, just restart your Mac until you succeed booting.

    Alternatively you may start from a bootable installer thumb drive (Mavericks or Yosemite) or a thumb drive containing a full system (Mavericks or Yosemite).

Adding GUID partition table and the already existing MBR partitions:

  • Open Utilities -> Terminal
  • Enter diskutil list to get an overview
  • Unmount your main disk with diskutil umountDisk /dev/disk0 (assumimg disk0 is the disk identifier of your main disk).
  • Add an GUID partition table with gpt create /dev/disk0
  • Add the EFI and the existent "MBR partitions":

    The command basically looks like this:

    gpt add -b StartBlock -i IndexNumber -s NumberOfBlocks -t Type device

    To add all your partitions enter with the data found in your fdisk list:

    gpt add -b 40 -i 1 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B /dev/disk0
    gpt add -b 409640 -i 2 -s 400094952 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk0
    diskutil umountDisk /dev/disk0
    gpt add -b 400504592 -i 3 -s 1269536 -t 426F6F74-0000-11AA-AA11-00306543ECAC /dev/disk0
    diskutil umountDisk /dev/disk0
    gpt add -b 401785650 -i 4 -s 67071375 -t EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 /dev/disk0

    The size of the last partition (your Windows partition) is odd because usually it's a multiple of 8. But I have no explication for this.

    The first partition is the EFI partition, the second your main Yosemite volume, the third volume is your Recovery HD and the last one is the Windows partition.

  • You may label your partitions with following command if they have no names:

    gpt label -i number -l name device


    gpt label -i 2 -l "Macintosh HD" /dev/disk0s2
  • Quit Terminal by entering quit and start Disk Utility

  • Check the consistency of your main volume (disk0s2)
  • Reboot to your main volume and install El Capitan

iPartition just payed for itself.. Convert function! Kd4hls

  • Welcome to Ask Different! Please refrain from adding comments in the Answer section, this is for answers to the questions. Once you have sufficient reputation you’ll be able to add comments and ask follow-up questions. To gain reputation, answer questions that are clear and concise. - From Review -
    – fsb
    Jun 19, 2016 at 23:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .