What username/password is the eDrive OS looking for?

I am unable to use the eDrive on my system because I can't log in to its OS. When I boot from the eDrive, it comes up to the screen with the username/password prompt, though with a red background, instead of the usual charcoal color. I was surprised it gave me a password prompt, as booting from Apple's recovery partition doesn't do that.

I'm not sure what username/password the eDrive's OS is looking for. None of the accounts running on the main partition work, including the administrator's account. It won't accept a blank username & password.

Suggestions? Thanks.

FWIW: iMac Retina 5K, OS 10.10.3, TTP 8.0.1.