Serial string readout analyzed and logged onto an SD card
There are a number of ways that this can be accomplished.


If the string is always of a fixed length and the characters are always in the same place (this may require packing), then you could:


pull the required sections out, one character at a time, using the char array (i.e. string) index. However, in this case, this is rather long-winded and clumsy solution
Similar to above, use strncpy() to copy contiguous characters, see Given a starting and ending indices, how can I copy part of a string in C? 
A more elegant method would be to use sprintf(), see How to copy a string into a char array in C++ without going over the buffer

Or you can use regular expressions (regexp) and patten matching. See How to read response from server using an Ethernet shield?
use sscanf, see Extract binary data from text string. Like Perl pack, not just regexp