0

I recently tried to install Ubuntu onto a USB Flash Drive and with the help of rEFInd, was hoping to triple boot my MacBook Pro, where in conjunction with MacOS, I had also installed Windows 10 through Bootcamp. Before the install, I used the excellent advice of Tim Richardson to logically disconnect my internal drive via flags so that my MacOS wouldn't be accidentally overwritten. I was able to successfully install Ubuntu onto the USB Flash Drive but unfortunately the installer changed my MBR to a Hybrid MBR, which stopped me from accessing Windows. Through the documentation provided by Rod Smith and his gdisk utility, I was able to resolve this issue, but not entirely. I have been reading a lot about Hybrid MBRs and the problems they can cause and I believe I am still not out of the woods. When I execute an fdisk on my internal hard drive, I get the following:

Disk: /dev/disk0    geometry: 30515/255/63 [490234752 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE    0   0   2 - 1023 255  63 [         1 -  490234751] <Unknown ID>
 2: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused

However, according to the documentation, the Protective MBR that Disk Utility creates on the Mac should look like this, which is what I had before the install:

Disk: /dev/disk0    geometry: 30515/255/63 [490234752 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE 1023 254  63 - 1023 254  63 [         1 -  490234751] <Unknown ID>
 2: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused

Does anybody know what utility I can use to correct this? I tried the command "fdisk -u /dev/disk0" to write a new MBR but that left things as is. I am not comfortable leaving my system like this and would consider doing a full restore to get back to the correct state. Thanks in advance for any and all comments.

0

I was able to restore the CHS (Cylinder/Head/Sector) in my partition map by issuing the following command:

diskutil repairDisk disk0

I'm not sure if it was absolutely necessary to do this as I read that "GPT knows nothing about CHS geometries" in Rod's documentation, but I feel much better now that it is back to its original state.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.