Here is an observation, after i logged in via SHH(Command in your terminal: ssh root@ipAddressOfYun) and went to the mnt folder (Command : cd /mnt) , the sd card was mounted as sda1.
Therefore i change the the following line in the example sketch from File dataFile = FileSystem.open("/mnt/sd/datalog.txt", FILE_APPEND);
to File dataFile = FileSystem.open("/mnt/sda1/datalog.txt", FILE_APPEND);
After this the file datalog.txt was successfully created in the path/mnt/sda1/, to check this get into the /mnt/sda1 folder (cd /mnt/sda1) and then type (ls -l)
Note: this was a SD card, that i was using in my Android phone.