本文发自 http://www.binss.me/blog/solve-sd-card-slot-read-only-problem-for-macbook/，转载请注明出处。
今晚准备往 Micro SD 卡中拷点东西，于是拿出买卡送的 SD 卡套，插入 macbook 侧面的卡槽中，结果发现无法写入。前后折腾了半个小时，在此记录。
文件系统不可写？
首先怀疑内存卡是不是被格式化成什么奇怪的文件系统导致 OSX 不可写。
但当我把 Micro SD 卡插入到 Micro SD 转 USB 的读卡器后，发现又可写了。因此排除该问题。
既然有读卡器，为何还要折腾这个？因为读卡器是 USB 2.0 的，速度太慢。
SD 卡套坏了？
于是我把 SD 卡套插到 Windows 笔记本中，发现可写。因此排除该问题。
SD 卡套设置成只读了？
无论我将卡套上的拨片拨到 lock 还是 unlock，都无法写入。
无奈之下，只能 google。在官网的论坛上发现了一篇比较扯的帖子：
https://discussions.apple.com/thread/2166984
说是要把拨片拨到 lock 和 unlock 中间，于是我照着干了：
发现果然可写了。。。为了探究背后的原因，我研究了下 SD 卡 lock 开关的原理：
In fact, the lock switch has no bearing on the internals of the SD card at all. If you rip an old SD card apart you will see that it is nothing more than a little plastic slider that rides in a groove, and does not affect the electronics of the card itself.
Its position is sensed by a small switch -- often just a spring-loaded little bit of metal -- inside the card slot on the card reader. When the switch is in the "lock" position it engages the switch in the reader and it tells your computer (or whatever device), "Hey, this thing is locked, don't write to it."
It is 100% up to the manufacturer of the reader to ensure that the lock functionality works and is supported. Many cheap Chinese knockoff card readers, especially those that do not fully encapsulate the card and leave it sticking part or most of the way out of the slot, don't support the lock switch at all. You can leave the switch in whatever position you want and it will have no effect.
按照这个说法，macbook 的 lock 触点比较奇葩，需要把拨片放到中间才能避免 lock 。
