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Hybrid MBR not entirely converted on MacBook Pro

Super User Asked by camty04 on December 5, 2021

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.

One Answer

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.

Answered by camty04 on December 5, 2021

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