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.
- The problem happens more often than not on SDHC type cards.
- Mac has some out-of-specification code to write to SD cards which is not compatible with other card readers.
- There is a fault with the onboard card reading which falsely reports that the physical read-only lock is active.
- It matters how you insert the SD card into the SD reader’s slot.
- All problems can be solved by using an external card reader.
/dev/disk1so I’ll use that in the work-around procedure that follows:
- Open a terminal window and type:
$ ls -l /dev/disk1* br–r—– 1 mrn operator 14, 0 11 Jun 00:24 /dev/disk1
- 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.
- Insert the card firmly applying the pressure directly along the line of insertion.
- Run the above line again to check the read-only status.
- 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.
- 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
Perfect - thank you!
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