Off line media issue

Good day,

Using HB Pavillon Series PC

Sony DCR-HC85 digital video camera

.avi is 720 x 480 wide screen 30 fps

Premiere Elements 12

I Have edited a number of programs with no issues except the occasional pop up screen asking where a certain file is before opening the current project.. I know typically that I have moved, deleted or renamed files or folders that where imported into the project..

This is something I can't get my head around to ask at one point where did I rename or move files? 

A little background, my wife and I went to Yellowstone Park in spring of 2017 wonderful trip.. Shot seven cassettes of tape, downloaded them to the program using several folders for convenience.. I had completed about 1 1/2 hrs. onto the  time line, all worked out well.  Pushed the "Save" button on the top right corner and closed out.  At a later time I would open up the program and continue to add to the time line, no issues.

I used an external H/D to backup program or project by clicking on the Adobe icon (purple "Pre")  and sent it to the ext. H/D.  This is where I think I messed up.

A Few days later opened up the project on my PC (not from the ext. H/D) and find everything on the time line is "off line" including audio.  All my work segments are there but no A/V.  I have had this issue before and by fluke re-linked the missing files but didn't make note of my steps.

Opening the program in the external hard drive gave me the same results. Basically I need to know how to properly back up my project.

Please use small steps in helping me to fully understand what is taking place so when it happens again it will be an easy fix.....sure!

Thank you for your reply,

Gil