Due to powerloss (and some failed revovery tries) /Datavolume had tons of errors on my WD mybook World 1 TB single drive. This thing is really hanging to the mounts, hard to do anything with the damaged partition. My solution (after few hours of reading and trying):
/Datavolume is at /dev/sda4. Some mounts are pointing to /dev/sda4 plus few services also using the partition (eg. samba mount points). You have to try getting rid of them. To see mounts use "mount" and "df -h"
If the data is not needed:
1. stop services that might use /dev/sda4 eg. /etc/init.d/samba stop
2. unmount everything using /dev/sda4 with umount -l if necessary
3. use parted to delete partition
4. recreate and format the primary partition 4 with mkpartfs to ext2 (that writes over data on the partition)
5. quit parted and reboot
6. mybook cannot mount the partition - but this is what we wanted
7. now format is to ext4 with mkfs.ext4 /dev/sda4
8 reboot, mybook mounts the fresh and clean /dev/sda4 ext4
9. recreate shares, don't forget to chown them
If the data is needed:
1. try to save anything worth saving
2. try to umount following steps 1-2 from the first section above
3. try to use fsck on /dev/sda4
4. if /dev/sda4 is still used try to force the unmount with umount -f
5. if the thing is still hanging to the partition, dismantle it and use a linux box to fsck the partition (sorry to say that but I have no better idea presently)
I suggest not to try fscking a live partition.