the tt read the 2 gb minisdcard but don't read 8gb minisdcard
did you transfer the content of 2 gb microcard to 8 gb card
did you download a new map or anything else that's need patching
when you insert the card what is hapening with your tt - do u see any splash screen or hear anthy.
I used a adapter and a microsd card and I have had no problem with my tomtomgo 730, (card formatted in fat32).
I remember with a new microsd card (original format), I had a problem with starting the gps and then I formatted with Windows. and since no worries.