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I have an existing binary file containing numpy array data. It has been created using open,fwrite & close and I can read the data using fread. I want to be able to either append a new array to the end of the file or update an existing array within the file. I've tried opening the file with a mode of either 'ab+' or 'wb+' and then writing the data using something like.... fd = open(vfname, 'ab+') if fd: filepos=(self.id-1)*self.yarray.size*4 fd.seek(filepos) fwrite(fd, self.yarray.size, self.yarray,'f') fd.close() When I use a mode of 'ab+' it looks like the data has been written to the file ok (no errors reported) but when I read it back I get my original data. When I use 'wb+' then my updated data gets written and read back ok. But when I reload the file, everything apart from my updated data (i.e. everything before it in the file) is now zero. The '+' in the mode seems to make no difference. What am I doing wrong? Thanks Bren.