It’s well-established that SD cards have a finite lifespan. This lifespan is dependent on the number of writes that you do to the card (although the number of reads is largely irrelevant). Alexander van der Sar has blogged about using a piece of software called Ramlog. At boot time, the current logs are read into memory, then subsequent log writes are done directly to RAM, therefore not using the SD card. The logs are then written at shutdown time to the SD card, thus minimising the number of writes. Read about this here.