How to mount a hard disk as read-only from the terminal

You do not mount '''/dev/sda''', that refers to the entire disk. You mount '''/dev/sda1''' or whatever partition you want. Make a mount point, call it anything you like.
sudo mkdir /media/2tb
Mount as Read Only
sudo mount -o ro /dev/sda1 /media/2tb
When your done, you should unmount the disk
sudo umount /media/2tb
See man mount or https://help.ubuntu.com/community/Fstab