Tried a new 8GB data card for above. Tried all possible ways suggested on forum to get new data card to work. Could not get TT520 AND tomtom home to work at the same time. If one works the other does not. Peculiar solution: 520 works fine but before connecting to TT home, release data card and put it straight back in. TThome then recognises it straight away and updates maps etc. Not an ideal solution but it seems to work.
As far as I know, the device is able to recognise only SD cards but not SDHC.
8GB "Micro" brand HC card and adapter
Seems to be random whether device it sees it or not. If you pop the card out and straight back in before connecting to TT home , it is recognised straight away by TT & apple. Peversely if you don't pop the card out first then my Mac does not recognise the TT520 at all. As soon as you disconnect the device it ask's if I want to connect to the computer again. If I say no, Apple still sees it. As soon as tt520 is physically disconnected - pop the card etc.
I think it might be a political power struggle, they might be sulking as to who is in charge of handling the card.