A portable storage device (MC) is disclosed, which comprises a memory (MEM) for storing data (DAT), a data interface (INT) for exchanging data (DAT) between the memory (MEM) and a host device (DEV), radio communication interface (RI) designed for receiving a key (K) from a transponder (T), checking means (COMP) for checking if a key (K) has a predefined value (V, and access inhibit means (SW) for controlling access to the memory (MEM), wherein the access inhibit means (SW) are controlled by the checking means (COMP). Access to the memory (MEM) is only granted if a certain key (K) can be received, which means that a certain transponder (T) has to be in the vicinity of the portable storage device (MC) for granting access. Furthermore, data (DAT) which is transferred from host device (DEV) to memory (MEM) can be encrypted and data (DAT) which is transferred from memory (MEM) to host device (DEV) can be decrypted. In this way for example commonly used memory cards can be secured against unauthorized use.