You need a new bootloader in order to recognise SDHC cards >2GB.
Originally Posted by britdub
Take out the SDHC card and keep it safe.
Put the 2 GB card back in and place a new bootloader on there. It should be at least version 5.5022 but several versions are tested and proven. Do not take a too new version as you cannot go back to an older version in case you would have to. Therefore stay away from versions 5.52** but rather use 5.5022, 5.5106, or 5.5128.
I prefer getting bootloaders from the source itself, but Tomtom never issued a newer bootloader for your device. So we download a navcore file for a newer Tomtom XL which does have said bootloader: Download this file and extract (winzip, 7-zip or Windows explorer) ONLY the file called system from it. The rest is a non-truck navcore for an XL. Copy the file system to your small SD card, check that the tomtom battery is properly charged and reboot your tomtom. It will flash the EPROM with the bootloader 5.5128 and reboot. After that, you can start up your tomtom with the new SDHC card which will be recognised.