I had the same problem with several cards but eventually found a way around it. DON'T buy a new one!
Make sure the SD card lock is 'off' and then use thin, opaque sticky tape (ie non see-through; eg insulation tape) to cover the whole gap but keep clear of the metal contacts at the back.
Your card should work fine now... don't know why this has to be done, but it works!
Additional info:
Reason #1, the mechanical lock switch accidentally moves to the "locked" position during insertion due to the poor quality of the switch. Hence, by taping it, you are actually preventing the lock switch to travel towards the lock position. i.e. downwards.
Reason #2, if the above fails, it is likely your SD card reader/writer's fault.
But don't discard it or buy a new one yet. Allow me to explain the mechanical design of the SD card lock mechanism. So that you may choose to repair it if you want to.
You will need a small paper clip and screw-driver to open your card reader. (Don't proceed if your card reader is still under warranty, please return/exchange with manufacturer)
There is a mechanical switch very much like a micro push button (I wish I could show you the picture/drawing). Anyway, depending on the position of the lock switch. You either push or release this micro push button inside the SD card reader. Due to mechanical wear and tear, or some forceful insertion of the SD card, the micro push button is in its "open" circuit position, i.e. write-protected. In order to make the SD card reader writable, we will need to "close" the circuit.
(Note: this is an irreversible process as I will be telling you how to remove the write-protection feature of your SD card reader. Once you do this, you would be unable to use the SD card lock switch to lock your data. So, just be careful not to accidentally delete your data)
To proceed, you need to remove the main circuit board from the SD card reader/writer enclosure.
The location of the micro push button switch is the same location as the unlock position of the SD card when inserted in the SD card reader/writer.
From the BACK of the exposed SD card reader, insert the unfolded paper clip (same way when you try to poke something like a device reset button) except, that it has to be long enough to allow the micro switch to touch together for contact. Then insert the SD card.
Once the contact is "closed", your SD card reader/writer can write again.
Taken from Wikianswers