Put it your fstab (File System TABle) and set the mount to "auto."
Here's a good description of fstab:
Note: In Ubuntu, you will likely see devices identified by Unique Identifiers (UIDs). You can use the tradition mount point lingo /dev/sdb1) along side the UIDs with no problems.
If you mess up the new entry, not to worry, the old ones will still work, so you can test. But it would be a good idea to make a backup first. It's always a good idea to back up a configuration file before editing it.
Edit: After mounting the external drive and before doing any editing, you can use the command cat /etc/mtab
to see how Ubuntu identifies that drive.