A diagnostic apparatus comprises a diagnostic data buffer constituting a volatile memory, and a non-volatile memory capable of receiving data from the buffer. A data buffer controller is also provided and is operably coupled to the buffer and has an event alert input and a data channel monitoring input for receiving diagnostic data. The buffer receives, when the state of a buffer status memory indicates that the buffer is in an unprotected state, at least part of the diagnostic data received by the controller via the data channel monitoring input to the buffer and the controller sets the state of the buffer status memory to indicate the protected state in response to receipt of an event alert received via the event alert input. A controller monitors the buffer status memory and copies a portion of the buffer to the non-volatile memory in response to the buffer status memory being set to be indicative of the protected state.