After upgrading to windows 8.1 I couldn’t able to mount windows partitions in ubuntu. Had to google it and finally deciphered the mystery behind fast booting time of windows 8.1. Actually it keeps all drives in hibernate mode even when you shutdown. That is why ubuntu finds it difficult to mount . To resolve this, you just run command as an administrator in windows and type “powercfg.exe /hibernate off” .
Warning : you are windows booting time will increase drastically