As mentioned above in reply #2, if you just copy the iPhoto library to -one- external drive (and delete it from your MBP), the files are NOT backed up.
You need ANOTHER COPY of the files, just in case. That means a SECOND external drive of some sort.
One way to "juggle" multiple backup drives is with a "dock" type of drive adapter. To see what these are, go to amazon and enter "usb3 sata dock" into the search box, and you'll get many hits. These are cheap and useful devices, starting at around $20.
You might use an external "enclosed" drive for your primary "archive" drive (the first drive to which you copy your photos). Keep that one near the computer.
Then, use a USB3/SATA dock to create at least one backup of the archive drive. Keep this one in the house, as well. This serves as the "immediate backup" of your "archive" drive.
If the pics are REALLY important to you, you probably want a SECOND backup, as well, stored "off-site" (different building than which the computer is in). What happens if there's a fire, or theft ?