Thread Subject: Need File I/O help for reading and rewriting string data

I need to read from a CSV file of mixed data types (mostly strings). I
need to process each record (such as adding a few more values to it).
Then I need to write (or append) the data to another CSV file.

I can't seem to find a decent way to do this. The code below is super
slow on large files (and this code is a drastic simplification of what
I really need to do). I actually have to look for and remove duplicate
records before writing out the modified data.

I would appreciate any suggestions for improving the code. (For now
I'm stuck using ASCII files, so I can't go to a database or binary
files.)

filedata = textread(myFilename, '%s','delimiter','\n');
charRec = '';
outputData = cell(length(filedata), 1);%preallocate
for k = 1 : length(filedata)
record = strread(char(filedata(k)),'%s','delimiter',',');
for m = 1 : length (record)
temp = strcat(record{m}, ',');
charRec = strcat(charRec, temp);
end%for
charRec = strcat(charRec, 'newData');
outputData(k) = {charRec};
charRec = '';
end

testFile = fopen (testFilename,'wt');
if testFile ~= -1
    for k = 1 : length(outputData)
     fprintf(testFile,'%s\n', char(outputData(k)));
    end
    fclose(testFile);
end


3 lines of sample data:
lastname1,firstname,DOB,Male,Caucasian,,,sp = 0 ti = 26,22,ox(1),03-
Aug-2007 10:36:48,13.15,5.85,189.058,18.9,5.8
lastname2,firstname,DOB,Male,Caucasian,,,sp = 0 ti = 33,22,ox(2),03-
Aug-2007 10:37:20,16.54,6.35,213.073,21.3,7.3
lastname3,firstname,DOB,Male,Caucasian,,,sp = 0 ti = 27,22,ox(3),03-
Aug-2007 10:53:16,15.86,7.68,192.082,19.2,8.2

I hope all this code and data comes out readable.