the empty quarter

Nautilus and Thunar sometimes refuse to mount NTFS disk (e.g. a USB hard drive) when it was previously not unmounted cleanly from Windows. It is still possible to mount it from terminal shell by using the following command:

sudo mount -t ntfs-3g -o force /dev/sdc1 /media/myntfsdisk

Where /dev/sdc1 is the device you want to mount and /media/myntfsdisk is the mount point.