I found the same thing. The message was not very helpful. It was saying the maps failed to download but in reality the card was just getting full.

I had to free up bunch of space on my internal card. I had plenty of space on my external card but it was ignored. I also deleted the partialy download maps directory and then had much better luck.

I have downloaded the eastern US.

I am finding the map data must be quite old also. The street I live on does not exist in their maps. The street has existed for at least 2 years. I moved into the house a year ago. The street already existed when we bought the lot about 20 months ago and had grass growing along the side walk. The street exists in google maps, virtual earth...