Installed El Captain and forgot the password

I've installed El Capitan in an external drive and I choose a password different from the one I use with Yosemite installed in my internal drive. Today I connected this external drive and restarted using it in order to install some applications, but when I was asked to enter my password I figured out that I forgot it and I cannot remember anyway.

Is there a way to workaround this situation without having to format the drive and install everything from start?