You have to mount it.
First issue "mount" command to see wheather it is already mounted.
If it is not mounted, create a directory. This is where you would access your drive. This could be something like /mnt/xp
Get to a terminal as root. Try this
mount /dev/sda1 /mnt/xp
Here I assume your device file for the USB hard disk is /dev/sda1
If everything went successfully, you would be able to view it under /mnt/xp
Note: See the following post by okmyx