recovery mode terminal password reset

I moved my old iMac onto an external HD after it died. I'm running os 10.11.6 My user password isn't working. It's an old password that I've used successfully for years without a problem. I want to reset the password now that it's no longer working...

I have a macbook air posing as my old desktop wile I wait for my new computer to come. I boot to the external HD. Aside from the password problem everything is happy.

I boot into recovery mode. Go to terminal type resetpassword my macbook user id pops up and I can change that one but not the one on my external drive.

I have the computer set to boot to the external as default.

Funny enough my password is the same on both computers. It still works on my macbook...

Any help? Thanks,