Microsoft Windows formatted drives ( NTFS) are normally auto mounted as READ ONLY
for your protection from windows viruses ,for one
i take it this is AUTO mounted and was not set by you in /etc/fstab
one way is to add a mount point in that file and set it "rw"
just google " fstab" and you will get more information than you will ever need
something like this
/dev/sdc1 /mnt/Windows ntfs-3g defaults 1 0
oops i was looking as the first (sdc1)