Ok I'll try and expalin but someone else probably can do it better. If you are writing numbers larger than bytes (8bits = 2^8-1 = 255) then you need to split the number up into separate bytes. I'll give a code example. I wanted to store the values of the arduino ADC in the memory card. Values from the ADC are 10bit (10bits = 2^10-1 = 1023). So the 10bit number needs to be split into two values to be able to be stored in 8bits.
// Store my 10bit value in an int (NOT a byte, which is only 8bits) so 16 bit number. High(1111 1111) Low (1111 1111)
int temp;
byte low; // Variable to store the low byte in
byte high; // Variable to store the high byte in
temp=analogRead(0); //Read in the value
// AND the number with 255 which wipes the 8bits at front of number
// The front 8 bits gets wiped->High(1111 1111) Low(1111 1111) = High(0000 0000) Low(1111 1111)
// So you can store half the number in the low byte variable
low=temp&0xFF;
// Rotate the number 8 bits right to get the high bits.
high=temp>>8;
So you can then store each byte in an array and then write it to disc. To read back from the disc you read in each byte and then join them together again.
byte low;
byte high;
byte info[2];
int value;
sd_raw_read(0,info,2); // Read the 2 values from disc
low=info[0]; // store low byte
high=info[1]; //Store high byte
result=high<<8; // rotate left 8 bits the high value and store in result
Serial.print(result+low,DEC); //Add low and high together to get the original number you split up into 2 bytes
The easiest way to see how this all works is to try this code and use Serial.print after each operation and print the numbers in binary. Then you can see how the bit manipulations are working. For larger values long int etc, you have to split the number up into 8bytes and then store each byte individually. So if you wanted to store a 32 bit number you would need to split it into 4 bytes and then write each byte to disc.
Well, hopefully I explained it ok 