Attempting to boot into Yosemite from external drive rejects password
I need to access old software which no longer runs under my current OS 10.13.6. It last ran under Yosemite. I have Yosemite installed on an external drive. When I attempt to boot from that drive I am asked for a password. The admin password for the computer is not accepted. Neither is my Apple id. When running in its normal operating mode I do not have the computer protected by a password: i.e. it boots up without any password being needed.
Can anyone suggest how I can get Yosemite to get through the password phase?
Many thanks in advance,
John
MacBook Pro 15″, macOS 10.13