Unlike on the Ubuntu Desktop edition, USB devices are not automatically mounted on Ubuntu Server. You will have to do it manually. Luckily it’s quite simple.
First, you need to find out which device needs to be mounted:
sudo fdisk -l
Most likely, your USB Hard Drive will be /dev/sdb1.
Now we create a mount point (directory) to which we want to mount the drive. For instance:
sudo mkdir /media/usb_hd
In many cases, USB Hard Drives are formatted with the NTFS filesystem, so the following command should work:
sudo mount -t ntfs-3g /dev/sdb1 /media/usb_hd
Your USB Hard Drive contents should now be available under /media/usb_hd.
When done, unmount the drive, and optionally remove the mount point (directory):
sudo umount /media/usb_hd
sudo rmdir /media/usb_hd
For more information, please check out the Ubuntu MountUSB help page.