I'm using micro sd with adapter for instaling maps on rns315.If you have SD card use this :Micro SD TO SD Card Extension Cable Adapter Flexible Extender SD/SDHC/MMC/.
To unlock/lock SD card without loss of data, it is possible to use an electric circuit with ATmega processor. Simply press the unlock button, delete deviceid file, press the lock button and it's done :-) It is also possible to read CID of locked/unlocked card in PC using this circuit. I build these circuits and send to the whole world...