It is mounted as rw ?! I have a 3TB formatted as NTFS connected without problems.
What format is your HD? Is it your computer that can not write to the drive all of a sudden? Are there any movies on there? Did it just stop working?
You should also be able to reach the HD while openELEC is running.
On your computer press leftwindowskey + r to bring up the run box, type \\ipaddressofrpi in the field and press ok.
A list of folders on the running RPi should appear. Try navigate to the media folder and your harddrive should be found inside. Try copy a file to it?