How to link from another course?
Go to the page for editing your course's content and select the option to add a link from another course (1).
Select the course (2) and the unit (3) you want to link.
From now on everytime you update the master course, all linked units will be updated automatically .
Note: Test, surveys, assignments and ITL units can not be linked! Also, a linked unit will not show up properly if cloned. You will need to clone the original unit.
The difference with cloned units
Linked units allows you to make a link to the original unit in order to use it to the new course. Every change to the original unit will be reflected to the linked units that will be automatically updated.
Cloned units allows you to clone a unit to the new course. Although they are sharing the same content, the original and the cloned unit are different and not related with each other. So any change you make to the original unit will not affect the cloned.