I spent almost half a day trying to figure out why the SD card plugged into the side of my MacBook Pro suddenly became read-only. After wading through an awful lot of Apple forums containing “are you sure you have the SD card read-only lock in the correct position” I discovered several aspects of this problem that will hopefully save other people from pulling their hair out.

1. The problem happens more often than not on SDHC type cards.
2. Mac has some out-of-specification code to write to SD cards which is not compatible with other card readers.
3. There is a fault with the onboard card reading which falsely reports that the physical read-only lock is active.
4. It matters how you insert the SD card into the SD reader’s slot.
5. All problems can be solved by using an external card reader.

The problem I was suffering from was that of the false positives on the SD card reader’s read-only sensor. The device file for the onboard SD card reader was /dev/disk1 so I’ll use that in the work-around procedure that follows:

1. Open a terminal window and type:

   $ ls -l /dev/disk1* br–r—– 1 mrn operator 14, 0 11 Jun 00:24 /dev/disk1

2. Make sure the SD card’s physical switch is pressed down in the unlocked position. There should be a label on the card itself to remind you.
3. Insert the card firmly applying the pressure directly along the line of insertion.
4. Run the above line again to check the read-only status.
5. If the device still reports as read-only, pull the card out and insert your fingernail in between the physical lock switch. The gap should remain when you remove your nail, then repeat the test.
6. Increase the width by about a fingernail’s worth each time you run the test and eventually you will reach the sweet spot. The test will report:

   $ ls -l /dev/disk1* brw-rw—- 1 mrn operator 14, 0 11 Jun 00:24 /dev/disk1

It took me about 3 or 4 iterations to find the right point.