SD Write Protect Switch is a Dummy

Need a physical write protect solution on a SD Card which will allow normal use when disabled and never allow data to be written when enabled? Well DON’T consider the SD card’s write protect switch since it does NOT do this. Not even Industrial SD Cards will allow guranteed cross platform write protect since it is subject to host system support - which is not common.

The SD Association spec for SD Cards calls for a “dummy” write protect switch which slides from an enabled or disabled position on the side of the card. The purpose of this switch was to replace the previous storage mediums like cassette tapes which had a plastic tab that could be broken off signifying to cassette recorders that the tape was “write protected.” To bypass this would only require something to physically fill the space in this cavity.

So What does the SD Card Write Protect Do?

The write protect switch is only a piece of plastic. There is no electrical switch attached to it inside the SD Card to provide a signal to the controller for enabled or disabled.

This SD write protect switch only serves a function if the host system checks the position. The original intent was for consumer electronic devices to operate similar to a cassette recorder by verifying which position the SD switch is set and either writing or not writing to the card.

This is fine for cassette tapes, but in practice, people moved the SD card from consumer electronic device which may have honored the write protect to a PC card reader, notebook or other device which was oblivious to the switch.

What a consumer thought was write protected would unknowingly be overwritten by this other system making the switch worthless. This is the main reason the SD write protect switch is rarely, if ever, used.

In fact, on later SD Association products such as miniSD and microSD, they do not have this switch defined.

Do Physical Write Protect Switches Exist on other Flash Storage Devices?

There are a number of flash storage devices which have physical write protect switches or jumpers. These are generally optional configurations by Industrial Grade suppliers.

Cactus Technologies designs and manufacturers CompactFlash, CFast and 2.5” SATA SSD with a physical switch or jumper option. In addition, Cactus provides a command based write protect on the CF, SSD and several other form factors.

If you would like to have a write protect option on another form factor or to discuss further, please contact us.

Steve Larrivee has over 30 year's experience in the data storage market, including 5 years at Seagate Technology and 10 years at SanDisk. He joined Cactus Technologies Limited as an equity partner and Co-Founded Cactus USA in 2007 with partner Tom Aguillon. Learn more about Steve on Google+