SB, try downloading in smaller sections. Kind of a pain, but it doesn't re-download anything you already have. You can also pay $5 for the option to register and download custom map types, such as Google Terrain (my favorite right now). Below are some notes and threads I have for myself
To download satellite imagery and USGS tops maps
http://www.outdoorgearlab.com/a/11168/Cheap-Motorcycle-GPS-How-to-Make-One-Using-an-iPhone-for-Under-150
http://tlcfaq.com/main/2013/08/motion-x-custom-maps/
http://www.bemental.me/blog/2015/2/19/custom-maps-with-motionx-gps
In the field below the name for your map source enter a map server URL. Remember to download it after you register it in Custom Map Types
http://services.arcgisonline.com/ArcGIS/rest/services/USA_Topo_Maps/MapServer/tile/[Z]/[Y]/[X]
http://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/[Z]/[Y]/[X]
http://mt1.google.com/vt/lyrs=p&x=[X]&y=[Y]&z=[Z]