I was getting exactly same error.
Just get a normal 32GB SD card, format it to FAT32, and then unzip the navi files and copy everything inside the SD's root. at the end try to upgrade it again.
There is nothing to "upgrade". It sounds like you're talking about the Navigation Plus system, which as a built-in hard drive. I'm talking about Media System Plus which includes the basic Navigation system. The maps are read from the SD card directly - "upgrading" is simply putting in an SD card with newer maps on it....which doesn't work.