You've got some great ideas - and I've tried them, but the problem is the Erase does not work for the DVD-RW, the disk shows on Disk Utility, but it is grayed out.
Did you have the same Mac, different Mac or PC to original burn those DVDs. Maybe you had and used something like an older version of Toast or iDVD when you original burned those DVDs?
So, we are going to get a a few 120 GB flash drive and put all our video's & pics on them. That will free up quite a bit of the MHD.
2 or 3 is a good idea! You should always have more than than just 1 copy saved some place. I always keep 2 or more backup copies backup of important things like photos and home movies.
I'm still not sure why the DVD-RW is not working properly, I am able to open the video's & pics but and copy videos & pics, not able to erase or copy over.
Maybe try Control (right) clicking the DVD image on your desktop and check to see if Erase is available in the pop-up menu.