There are ready made solutions:
Logomatic v2 Serial SD Datalogger - WIG-10216 - SparkFun Electronics
Or, roll your own: off load serial data recording to dedicated 328P and SD card - Storage - Arduino Forum
There are ready made solutions:
Logomatic v2 Serial SD Datalogger - WIG-10216 - SparkFun Electronics
Or, roll your own: off load serial data recording to dedicated 328P and SD card - Storage - Arduino Forum