This device was developed by Karl Lunt back in 2013, then improved by Nephiel in 2014. In 2018, I've designed and started manufacturing SD lockers, and have since sold a good amount of these. I've been asked to make a smaller, microSD-only version that doesn't need a microUSB cable for power, so, here it is!

This product is fully open-source - hardware and software sources are available through the links below.

Why use it?

• Make read-only Raspberry Pi SD cards
• Make virus-resistant LiveCDs (or, rather, LiveSDs) for all your computer maintenance needs
• Distribute SD cards with promotional materials
• IT forensic research and data recovery (reading from the SD card while preventing all write operations)
• Test your SD-card-powered products for unexpected behaviour (an SD card becoming read-only is a popular failure mode and tends to happen when the card controller detects severe data corruption).

How does it work?

• Plug the device into a USB port
• Plug in the SD card - the device will power up
• LED lights on boot for a couple of seconds, then shows the card's lock status (not lit = unlocked, lit = locked)
• Press the button to toggle the card's lock state
• By default, the TMP lock bit is toggled - it allows to unlock the card.
• To toggle the PERM bit (and make the card permanently read-only), hold the button while plugging the SD card in - it will flash the SOS pattern on the LED (and won't proceed until you release the button). This is done so you don't toggle the PERM bit by accident, which irreversibly switches your card to write-only
• Once you unplug the card, the device will power down. If you used PERM locking, don't worry - it will use TEMP locking on next card insertion (unless you hold the button again while inserting a new card).

Why does it work?

Larger SD cards have this mechanical "write protect" slide switch on the side, that you can switch and the card reader can check. However, that switch is more useless than useful, since 1) not all readers support it 2) the OS can ignore it 3) it obviously doesn't apply to MicroSD cards 4) the switch tends to slide accidentally (or fall out of the SD card altogether) when you have no intention of enabling write protection.

There exists a much more reliable method to lock the SD cards - the one this device is designed to use. The controller chips inside the SD cards (of all sizes, whether full-size or MicroSD) are required by the SD card standard to support a low-level command that locks the card into a read-only mode, preventing any changes - either temporarily (can be switched back to read-write) or even permanently (without a way to ever restore the write capability). This device has an ATTiny85 chip can send such a low-level command - this chip connects to the SD card, reads the lock status, shows it using the LED and allows you to toggle the lock status using the button. It supports both temporary and permanent SD card locking, too! Don't worry, I designed the permanent lock capability to be hard to trigger on accident.

Support

• Only microSD cards
• SDSC (regular SD cards <2GB), SDHC and SDXC cards - I've tested all three types

ATTiny45 option

There's an option for you to get a SDLocker with ATTiny45 instead of ATTiny85 if you want to save a bit of money - the downside is that if you want to reprogram the SDLocker later to use i.e. a password lock firmware, there's no guarantee that it will fit in the ROM and/or RAM of the ATTiny45. However, it will work with stock TEMP&PERM lock firmware just fine.

Disclaimer

Each device is tested by me (on both SDSC and SDHC cards) before I send it out, and is designed so that you don't accidentally PERM-lock the card. Having taken these precautions, I do not take any responsibility for any SD cards you accidentally lock permanently, or any SD cards that may break as a result of using this device.