Electronic – While loop optimized away

Smaller capacity SD cards will often perform better for single-block writes, because their erase page size is smaller. When you write to the card, even if you only write a single byte, the card's internal controller may have to read an entire page (often 4K) into a buffer, modify your one byte, erase the Flash page, then write the whole thing back to the Flash memory. So cards with a smaller erase page size will have less of a penalty for small writes at random addresses.

What microcontroller are you using? SdFatLab works on AVRs and does support multiple block read/write operations. Your files must be in contiguous blocks on the SD card to take advantage of it.

Check out some of the discussion and benchmarks at http://elm-chan.org/docs/mmc/mmc_e.html

You can use these macros that get defined automatically when you include <avr/io.h>:

SIGNATURE_0
SIGNATURE_1
SIGNATURE_2

For ATmega1280, they're defined as:

/* Signature */
#define SIGNATURE_0 0x1E
#define SIGNATURE_1 0x97
#define SIGNATURE_2 0x03

in iom1280.h (which is automatically included through <avr/io.h> when you compile code for the m1280)

For example, this will send the three bytes via UART:

uart_putc(SIGNATURE_0)
uart_putc(SIGNATURE_1)
uart_putc(SIGNATURE_2)

If you truly want to read the fuses, you'd need to use boot_signature_byte_get macro from <avr/boot.h>