I recently downloaded ubuntu server 12.10 and created an install disk. I then proceeded to boot into the disk and install the server onto an external usb flash drive. After the installation was complete it ejected the disk and rebooted the computer as normal but through all my efforts i cant boot into the flash drive. I am running a 2011 macbook pro with mountain lion, 4gb of ram, 1tb hard drive, and an intel I7 processor
Try restarting and immediately press and hold the Option key to access Startup Manager, then select your boot device.
Try different usb ports if that doesn't work. Some USB devices have better compatibility than others.