Bootcamp Partition MBR Needs Repair... "Operating system missing" During Boot

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Level 1

4 points

Bootcamp Partition MBR Needs Repair... "Operating system missing" During Boot

Hi, 1st post here...

My 2011 MacBook Air's High Sierra OS is bootable, but the Windows 10 partition's MBR entry got wiped accidentally, and so it needs to be rewritten into the MBR (I'm getting the dreaded "Operating system missing" when I select the Windows drive during boot).

I attempted (for the first time) to do so(repair the MBR) in Terminal but got the warning below;

"Warning! Main Partition table overlaps the first partition by 33 blocks! You will need to delete this partition or resize it in another utility."

An online search showed that some folks got similar warnings but with a different number for the partition and number of blocks(56?). One such person used GParted to reduce the last partition by like 3MB and apparently the warning went away.

Therefore, I got myself setup with a bootable USB GParted drive, and got more comfortable with the Terminal process(though I was using the Terminal not in Recovery Mode, but just in a regular OSX session).

1.) Is it possible to perform the Terminal process for repairing the MBR in a regular OSX boot session(not Recovery Mode)?

I ask because currently my system does not have a functioning Recovery Partition, so I've been using a bootable USB High Sierra Installer drive to access the Recovery Mode(quicker than waiting forever for the online method), but in this case the Disk Utility's Terminal keeps indicating "command not found".

2.) In the Terminal's warning mentioned above, what partition numbers are the "Main partition table" and "first partition" exactly?

3.) Is there a way to check where the "33 blocks" overlap? If so,

4.) Is it possible to visually identify in GParted, where the problematic overlapping occurs?

If I can get some help in figuring out which partition to adjust, I could perhaps shrink a minute amount of the end of the "front partition"(of the 2) in GParted, then proceed with Terminal operations to repair the MBR to include reference to the Bootcamp partition.

Thanks...

P.S. My recent trial Terminal session below in "additional text".

(BTW, in GParted, partition no.3 shows as "BOOTCAMP", though in Terminal it shows as Windows_FAT_32)

Terminal Session

MacBook Air 13″

Posted on Oct 8, 2025 2:19 PM

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Answer 1

omygot Author

Level 1

4 points

Oct 13, 2025 11:24 PM in response to omygot

Before poking deeply via Terminal, GParted, TestDisk etc., I installed a free EaseUS recovery app (older version for High Sierra) and recovered 1.5GB of critical data from the "invisible" BOOTCAMP drive, using the Mac OS.

1) Regarding "Warning! Main partition table overlaps the first partition by 33 blocks!",

I used GParted to create a space of 2MB between the Mac(#2) & BOOTCAMP(#3) partitions which didn't work, nor did a space between the Recovery(#1) & Mac(#2) partitions.

BUT a space in front of the 1st partition DID resolve the issue (i.e. got BOOTCAMP partition written as a boot partition in gdisk). Then, BOOTCAMP was visible in Mac OS, but it still didn't boot.

2) I used TestDisk to rewrite BOOTCAMP partition's boot sector & MBR. The commands proceeded successfully, but still no boot.

I backed up the 100GB BOOTCAMP partition, reinstalled High Sierra, Windows 7(Bootcamp Assistant) then Windows 10 via ISO.

In hindsight, the cause of the problem was seeded around 2018, using an EaseUS Partition app to shrink the Mac partition to boost BOOTCAMP's partition AFTER installing Windows (i.e. gdisk info showed references to "EaseUS Partition Master" for BOOTCAMP partition). Back then, I was unaware of the potential instability of the hybrid MBR (i.e. if the partition table "gets out or sync" due to corruption of the MBR). It took about 7 years for the partition table to take a dump.

This time I'm not shrinking the Mac partition, though it's 57GB (larger than I need as High Sierra is only like 14GB), since this will keep the partition table more stable, and I'll have more elbow room to play around in MacOS if needed.

I'm now fully restored... Thanks & Cheers~

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Answer 2

daacrone

Level 1

12 points

Oct 13, 2025 5:39 AM in response to omygot

Try using the command fdisk -e /dev/disk0 in the macOS Terminal. Enter edit 3 to change the partition type to 07 (NTFS). Enter write to force the changes and repair the MBR. Finally, enter quit to exit.

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Answer 3

doukl

Level 1

8 points

Oct 10, 2025 12:56 AM in response to omygot

macOS's built-in diskutil and fdisk commands are designed for GPT partitioning, and they don't handle MBR repair effectively, especially on bootable Windows partitions.

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Answer 4

Ladybang

Level 1

20 points

Oct 13, 2025 2:54 AM in response to omygot

Because your Mac is bootable in High Sierra, but the Windows partition's MBR is broken, careful resizing and partition adjustments are critical.

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