USB Hard drive mounting as Read-Only

I've been having a problem with my Raspberry Pi: it keeps mounting my USB hard drive as read-only. I've been mounting it with sudo mount -t ntfs -o umask=000 /dev/sda1 "/media/My Book" but for some reason it now says mount: warning: /media/My Book/ seems to be mounted read-only when I do so, and ls -l shows it's correct. The drive has its own power supply and it works fine with my laptop, so I think the problem must be with the Pi. What am I doing wrong?

Just to be clear, that command used to work, but now it doesn't. And it's now read-only for EVERYONE, even root:

$ ls -l /media
total 8
dr-xr-xr-x 1 root root 4096 Apr 28 11:11 My Book

The contents appear the same way.

Update: Probably found the cause. From /var/log/syslog:

May 17 19:37:52 raspberrypi kernel: [   64.688724] NTFS driver 2.1.30 [Flags: R/W MODULE].
May 17 19:37:52 raspberrypi kernel: [   65.195397] NTFS volume version 3.1.
May 17 19:37:52 raspberrypi kernel: [   65.195439] NTFS-fs error (device sda1): load_system_files(): Volume is dirty.  Mounting read-only.  Run chkdsk and mount in Windows.

Probably should've tried looking there before…