I see you put an issue on Blushingboy.net. I hope when it's fixed it corrects the strange behavior I see when writing to MMC with this lib.
In the meanwhile, there is a simple workaround that allows you to maximize the number of lines written to a file. If you write lines of fixed length l (including newline character(s)), the bug will hit when lcm(l, 512) (lcm = least common multiple: Least common multiple - Wikipedia) bytes will have been written to the file.
If you choose an odd number for l, then lcm(l, 512) = l*512: you will be able to write 512 lines to the file, which might be enough for some applications (including mine).