seacloudstudio,
if you’ve never installed Mac OS X on your external disk, then it won’t be bootable. (The “yes under bootable” is probably in reference to your internal disk.)
Your Early 2011 MacBook Pro has a FireWire 800 port, located between its Ethernet port and its Thunderbolt port. It requires its own cable to be able to connect to FireWire 800 peripherals or to another Mac with a FireWire 800 port. If you have a friend with such a Mac, you could purchase a FireWire 800 cable to boot your Mac into Target Disk mode, which would let your Mac act as an external hard drive to the second Mac; you could then use the second Mac to copy the photos and important files from your Mac.
How much free space is on your external disk? If you still have the grey Mac OS X Install DVD which came in the box with your Mac, you could install Mac OS X from that DVD onto your external disk, and then use Startup Manager to boot from your external disk, and then copy your photos and important files from your internal disk to your external disk using the Finder.