How can I mount a removable hard disk and make it can be written?
 

My system is Debian 4.0. This system can mount USB(fat) automatically and makes the disk writable. But for the removable hard disk(NTFS),it can mount automatically, but I cannot write on the disk. How can I figure out this problem?

The default NTFS driver in Debian doesn't support writing to NTFS partitions. You can install the "ntfs-3g" driver by running (as root):

apt-get install ntfs-3g

You then have to edit your /etc/fstab accordingly. More info can be found on www.ntfs-3g.org.

Generally you can use '-remount' option for the mount command to re-open a device in writeable mode. As already said, the requested mode has to be supported by the device driver for the given filesystem (especially for NTFS).