I just tried building a mapset with 2989 MB, and it still fails. Funny thing this time, it doesn't provide any sort of error message, instead it simply shuts off in the middle of loading maps.
I'm loading city navigator and an older topo mapset, so the older topos are smaller regions. Total maps 3,337. It takes about 24 hours to build the mapset using mapsource, so every time I try making it smaller its a bit of a while to test.
It´s not the size of the file - maybe the 8GB-card is to big. I have a 4GB card and loaded it with about 3,6GB map and it works fine.
I´m guessing you´ve got trouble with either a too big data-card for the device or too many waypoints (+500), which gave me some trouble - and a replacement of the device by Garmin.