Old iMac Hard Drive not readable as external
Do a cold boot and enter the password for the encrypted drive at that time. FileVault 2 doesn't allow access to encrypted drives after boot if you don't provide the password then.

When FileVault setup is complete and you restart your Mac, you will use your account password to unlock your disk and allow your Mac to finish starting up. FileVault requires that you log in every time your Mac starts up, and no account is permitted to log in automatically.

See: https://support.apple.com/en-us/HT204837