前言
我有一台Mac，装有OS X和Windows两系统，因Windows和OS X都能读写exFAT分区，
故若在Machintosh HD和Windows HD之间开辟一个exFAT分区，可以作为共享文件区。
我在OS X中用磁盘工具对Machintosh HD进行分区，在Machintosh HD和Windows HD之间开辟一个exFAT分区。
重启启动时，我按住了option，之后出现的启动选项中没有Windows。这因为刚才给磁盘分了区才导致的。
解决
重启MAC ，按住COMMAND+R进入恢复模式，打开Terminal
csrutil disable #关闭rootless
修改完，重启MAC，打开终端
sudo fdisk -e /dev/disk0
yanghaodeMacBook-Pro:~ yanghao$ sudo fdisk -e /dev/disk0
Password:
fdisk: could not open MBR file /usr/standalone/i386/boot0: No such file or directory
Enter 'help' for information
fdisk: > p
Disk: /dev/disk0 geometry: // [ sectors]
Offset: Signature: 0xAA55
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
: EE - [ - ] <Unknown ID>
: AC - [ - ] <Unknown ID>
: AB - [ - ] Darwin Boot
*: - [ - ] HPFS/QNX/AUX
fdisk: >
修改分区表
fdisk: 1> p
Disk: /dev/disk0 geometry: // [ sectors]
Offset: Signature: 0xAA55
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
: EE - [ - ] <Unknown ID>
: AC - [ - ] <Unknown ID>
: AB - [ - ] Darwin Boot
*: - [ - ] HPFS/QNX/AUX
fdisk: 1> setpid 4
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
*: - [ - ] HPFS/QNX/AUX
Partition id ('' to disable) [ - FF]: [] (? for help)
fdisk: 1> flag 4
Partition marked active.
fdisk:*> p
Disk: /dev/disk0 geometry: // [ sectors]
Offset: Signature: 0xAA55
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
: EE - [ - ] <Unknown ID>
: AC - [ - ] <Unknown ID>
: AB - [ - ] Darwin Boot
*: - [ - ] HPFS/QNX/AUX
fdisk:*1> write
Device could not be accessed exclusively.
A reboot will be needed for changes to take effect. OK? [n] y
Writing MBR at offset .
重启电脑按OPTION
