How to auto-mount a secondary Encrypted HDD on boot?
I don't know of a good way to make it automount at startup, but you can make it automount for any user by transferring the saved password from your user login keychain to the System keychain.


Open Keychain Access.app (it's in /Applications/Utilities)
Find the saved volume password. You can use the search field in the top right of the window to find it.
If the keychains list isn't visible (in the left-side sidebar, near the top), choose View menu > Show Keychains.
Drag the saved volume password into the System keychain in the sidebar. It'll ask you to authenticate twice (once as an admin, to modify the System keychain, then as your user, to get access to the user keychain item).


There's probably a way to  trigger the automount at startup, rather than at first login, but it sounds like first login should be good enough for your needs.