I am pretty sure I read that the purchased versions keep the old icon and the rental versions have the new icon and both can be installed on the computer simultaneously.
Also your backup sizes may be related to background rendering. I would do all basic cuts first then go back and add any transitions, effects or color grading to the project.
You could try turning off background renders and I would definitely say working on SSD drives is much better than even a 7200rpm spinning drive. I would double check the format of the external drive just to be sure. It does seem it is formatted for apple when I searched them. One thing I have read is that AFPS can start to cause some read speed issues over time when used on spinning hard drives.