I Googled a bit and i found this here:
Strange Error Message: Not Enough SpaceCode:Please Login or Register to see the links
With a custom map that I downloaded (Desert Tracks), MapSource gave me a strange error message. Apparently, there wasn't enough space on my 8GB card for a tiny 3MB map! If you use Mapsource, you will see this message regularly:
"This mapset is approximately xx MB, but only yy MB is available in the destination. Please select fewer maps and try again."
Typically, xx is very small and yy is very large. After you get the hang of it, you start to appreciate the humor of Garmin software engineers. So I gave up trying to transfer to the unit. Instead, I asked MapSource to transfer the mapset to a USB key, then I took the img file from the key and copied it to the GPS. It worked fine. I also tried to ask Mapsource to transfer the mapset to an SD card plugged into the laptop. It worked too.
It looks a nice workaround for this bug