SD cards can have have up to 128kB in one memory page. It doesn't write to the same page all the time but advance to the next free page with each write. It continues to use the next page till it finds the end of the memory area and starts to write from the beginning of memory again. That's one cycle.
A 8GB SD card has 64.000 pages, if they are of a 128kB size, it will use 4 pages for that 500kB 30 sec audio sample. If you continuously record it will take 16.000 writes of 4 pages to reach the end of a 8GB SD card, that's one write cycle. Some SD manufacturers state something like 100.000 write cycles of lifespan for one memory page. It should be impossible to wear out a SD card.
Sandisk has a 30 year, or lifetime, warranty for their SD cards as the number of maximum writes are so high that it would not wear out during your lifetime of recording HD video. Different speed classes of SD cards are defined at writing speed of around 50MB per second but in a scanner you have less than 1MB per second when reading data so high speed SD cards are not necessary.
Your only critical specification for a SD card for scanner use could be operating temperature and the quality of manufacturing. Both low quality SD cards and those specified as high quality are failing but it is not from wearing them out by too many writes to the SD card. Always have one spare SD card so you don't have to run out and buy a new one and also keep an updated copy if you cannot reprogram one easily, like when you are in a vehicle.
/Ubbe
