This is one from the archives, posted mostly for posterity.

This issue seems have been fixed sometime after 1.5, however I ran into it the other day whilst upgrading an EdgeMax Lite from an ancient version of the firmware (1.2). Luckily, the fix is quite simple:

Check if there is more than one image installed as this can cause issues with disk space

 show system image 

If there is a second image that is no longer needed

 delete system image 

Download the new image to a local machine and then copy via SSH

 scp ~/Downloads/ER-e100v2.0.8.tar 10.0.0.1/tmp/ 

Add the uploaded image to the system

 add system image /tmp 

Reboot the router to tidy everything up

 reboot 

It might then be worthwhile to delete the old system image if the upgrade was successful.