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.
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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