Installed Mountain Lion on ext. drive but doesn't show up on boot

[petrucci666]

I hold the Option key on boot but only Macintosh HD and Recovery HD show up, no external drives.

What am I doing wrong?

 

[simsaladimbamba]

Does it show up in System Preferences > Startup Disk when booted from the OS on the internal HDD?
I guess, the external HDD is properly formatted?

How to format a new HDD/SSD to install Mac OS X or make a bootable copy onto​