内置读卡器卡槽触点有问题，是老款 Macbook Pro 的通病了。以前遇到这个问题解决过，很久没用了又忘记，再次搜索找到一篇，转载备忘。
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 。