Manjaro, by default, requires a password to mount additional internal hard drives. To automatically mount a hard drive or specific partition at boot, follow these steps:
Open Gnome Disk. Select the desired hard drive (and partition if applicable). Click on the small cog wheel  to open the pop-up window. In the pop-up window, select “Edit Mount Options”. A new window will appear; uncheck “User Session Default” (the top option). Restart Manjaro for the changes to take effect. After rebooting, your secondary hard drive should automatically mount without requiring an admin password during user sessions. Because of features like this one, Gnome is the only desktop environment DE I use.