# 如何還原一 GUID 磁碟分割表之磁碟的 Hybrid MBR 設定？<br>How to Undo a Hybrid MBR Configuration on a GUID Partition Table Disk?
## 問題概覽<br>Problem Overview

# gdisk <disk device file>
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: hybrid
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with hybrid MBR; using GPT.

You want to revert a previously made Hybrid MBR configuration GUID partition table disk back to standard configuration to prevent programs not handling them well to misfunction(like Microsoft Windows installer)

## 解決方案<br>Solutions
### Using GPT fdisk (a.k.a. gdisk)
In fact `gdisk` *does* support replacing the hybrid MBR to proper GUID Partition Table's protective MBR

> "You can replace a hybrid MBR with a legal protective MBR by using the "n" option on the experts' menu." -- by Rod Smith, GPT fdisk's author

Simply press `x` in the `gdisk` interactive shell to enter the experts menu, press "n" the create a new protectie MBR and press "w" to write the changes to disk(and of course, press "q" to leave `gdisk`)

### Modify MBR Externally Then Put It Back In(obseluted)
1. Use `dd` command to make a copy of the MBR

    # dd if=<block device file of the specific disk> of=mbr.bin bs=512 count=1
    [sudo] password for Lin-Buo-Ren: 
    輸入 1+0 個紀錄
    輸出 1+0 個紀錄
    512 bytes copied, 0.00451046 s, 114 kB/s

1. Open the copied MBR file using `fdisk`, this time it won't consider it as GUID partition table

    # fdisk mbr.bin

    Welcome to fdisk (util-linux 2.27.1).
    Changes will remain in memory only, until you decide to write them.
    Be careful before using the write command.


    命令 (m 以獲得說明)： p
    Disk mbr.bin: 512 B, 512 bytes, 1 sectors
    Units: sectors of 1 * 512 = 512 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disklabel type: dos
    Disk identifier: 0x00000000

    所用裝置   可開機     Start    結束    磁區    Size Id 類型
    mbr.bin1                  1      2047      2047 1023.5K ee GPT
    mbr.bin2               2048    391167    389120    190M  c W95 FAT32 (LBA)
    mbr.bin3   *         393216 195702783 195309568   93.1G  7 HPFS/NTFS/exFAT
    mbr.bin4          195702784 937701375 741998592  353.8G 83 Linux

1. In `fdisk`'s interactive shell, remove all partitions that not in type 0xEE(which is the indicator that this is a GUID partition table's protective MBR)

    命令 (m 以獲得說明)： d
    分割區編號 (1-4, default 4): 2

    Partition 2 has been deleted.

    命令 (m 以獲得說明)： d
    分割區編號 (1,3,4, default 4): 3

    Partition 3 has been deleted.

    命令 (m 以獲得說明)： d
    分割區編號 (1,4, default 4): 4

    Partition 4 has been deleted.

1. In `fdisk`'s interactive shell, write the result back to the external MBR and leave `fdisk`

    命令 (m 以獲得說明)： w
    The partition table has been altered.
    Syncing disks.

1. Write the modified MBR back to the disk using `dd`

    # dd if=mbr.bin of=<block device file of the specific disk> bs=512 count=1
    輸入 1+0 個紀錄
    輸出 1+0 個紀錄
    512 bytes copied, 0.000167607 s, 3.1 MB/s

1. Profit!

    # gdisk /dev/sda
    GPT fdisk (gdisk) version 1.0.1

    Partition table scan:
      MBR: protective
      BSD: not present
      APM: not present
      GPT: present

    Found valid GPT with protective MBR; using GPT.

## 參考資料<br>Reference
* [\[Gptfdisk-general\] Support reverting a hybrid MBR configuration](https://sourceforge.net/p/gptfdisk/mailman/message/36060568/)
* [Managing EFI Boot Loaders for Linux: CSM: The Good, the Bad, and the Ugly](http://www.rodsbooks.com/efi-bootloaders/csm-good-bad-ugly.html)