Use the 'a', 'a+', or 'A' flags in fopen to append data. This should work for text files as well. You don't really need the 't' flag, as that only modifies the newline convention (which all proper text editors should display as expected anyway). Only if you insist on using the normal notepad on Windows (without any modifications) you will have to use use \r as well.
FOPEN Open file. FID = FOPEN(FILENAME) opens the file FILENAME for read access. FILENAME is the name of the file to be opened. FILENAME can be a MATLABPATH relative partial pathname. If the file is not found in the current working directory, FOPEN searches for it on the MATLAB search path. On UNIX systems, FILENAME may also start with a "~/" or a "~username/", which FOPEN expands to the current user's home directory or the specified user's home directory, respectively. FID = FOPEN(URL) opens the file at a remote location specified via the URL (uniform resource locator) for read access. The URL must be specified as a full path. For example, to read a binary file from Amazon S3 cloud specify the full URL for the file: s3://bucketname/path_to_file/example.bin For more information on accessing remote data, see "Work with Remote Data" in the documentation. FID is a scalar MATLAB integer valued double, called a file identifier. You use FID as the first argument to other file input/output routines, such as FREAD and FCLOSE. If FOPEN cannot open the file, it returns -1. FID = FOPEN(FILENAME,PERMISSION) opens the file FILENAME in the mode specified by PERMISSION: 'r' open file for reading 'w' open file for writing; discard existing contents 'a' open or create file for writing; append data to end of file 'r+' open (do not create) file for reading and writing 'w+' open or create file for reading and writing; discard existing contents 'a+' open or create file for reading and writing; append data to end of file 'W' open file for writing without automatic flushing 'A' open file for appending without automatic flushing FILENAME can be a MATLABPATH relative partial pathname only if the file is opened for reading. You can open files in binary mode (the default) or in text mode. In binary mode, no characters get singled out for special treatment. In text mode on the PC, the carriage return character preceding a newline character is deleted on input and added before the newline character on output. To open a file in text mode, append 't' to the permission specifier, for example 'rt' and 'w+t'. (On Unix, text and binary mode are the same, so this has no effect. On PC systems this is critical.) If the file is opened in update mode ('+'), you must use an FSEEK or FREWIND between an input command like FREAD, FSCANF, FGETS, or FGETL and an output command like FWRITE or FPRINTF. You must also use an FSEEK or FREWIND between an output command and an input command. Two file identifiers are automatically available and need not be opened. They are FID=1 (standard output) and FID=2 (standard error). [FID, MESSAGE] = FOPEN(FILENAME,...) returns a system dependent error message if the open is not successful. [FID, MESSAGE] = FOPEN(FILENAME,PERMISSION,MACHINEFORMAT) opens the specified file with the specified PERMISSION and treats data read using FREAD or data written using FWRITE as having a format given by MACHINEFORMAT. MACHINEFORMAT is one of the following: 'native' or 'n' - local machine format - the default 'ieee-le' or 'l' - IEEE floating point with little-endian byte ordering 'ieee-be' or 'b' - IEEE floating point with big-endian byte ordering 'ieee-le.l64' or 'a' - IEEE floating point with little-endian byte ordering and 64 bit long data type 'ieee-be.l64' or 's' - IEEE floating point with big-endian byte ordering and 64 bit long data type. [FID, MESSAGE] = FOPEN(FILENAME,PERMISSION,MACHINEFORMAT,ENCODING) opens the specified file using the specified PERMISSION and MACHINEFORMAT. ENCODING specifies the name of a character encoding scheme associated with the file. It must be the empty character vector (''), empty string (""), or a name, or alias for an encoding scheme. Some examples are 'UTF-8', 'latin1', 'US-ASCII', and 'Shift_JIS'. For common names and aliases, see the Web site http://www.iana.org/assignments/character-sets. If ENCODING is unspecified, or is the empty character vector (''), or is the empty string (""), MATLAB's default encoding scheme is used. [FILENAME,PERMISSION,MACHINEFORMAT,ENCODING] = FOPEN(FID) returns the filename, permission, machine format, and character encoding values used by MATLAB when it opened the file associated with identifier FID. MATLAB does not determine these output values by reading information from the opened file. For any of these parameters that were not specified when the file was opened, MATLAB returns its default value. The ENCODING is a standard character encoding scheme name that may not be the same as the ENCODING argument used in the call to FOPEN that opened the file. An invalid FID returns empty character vector ('') for all output arguments. FIDS = FOPEN('all') returns a row vector containing the file identifiers for all the files currently opened by the user (but not 1 or 2). The 'W' and 'A' permissions do not automatically perform a flush of the current output buffer after output operations. See also FCLOSE, FERROR, FGETL, FGETS, FPRINTF, FREAD, FSCANF, FSEEK, FTELL, FWRITE. Documentation for fopen doc fopen Other functions named fopen i2c/fopen icinterface/fopen serial/fopen