Ubuntu allows you to access removable drives, but I have a USB 2.0 External Hard Disk that says it is NOT removable (thanks, Iomega.) You may be in the same boat here. If the hard drive doesn't show up as removable you're going to need to edit /etc/fstab to add a mount point for it, or mount it from the command line.
If you need to use it once, just mount it as root (with a reasonably unrestricted umask, say 000).
If you need to use it multiple times, eeither /mnt or /media is an acceptable place. Once you add the fstab entry (do it as root, and back the file up first) you'll be good to go.
Another option: If the file system is actually mounted read-write, use root to make a folder on the drive with the appropriate permissions, and have your user accounts write to that folder.