What is the bootloader version of your Tomtom and what is the type (SD, SDHC) an size of the memory card?
For SDHC cards you need at least bootloader 5.5022.
Updating the bootloader is done by placing the payload file (called "system") in the fixed memory of the Tomtom or on a SMALL (<2GB) SD card and rebooting the tomtom. Be sure that your Tomtom is fully charged before attempting this because a power outage in the midst of a bootloader upgrade means a bricked Tomtom.
Once that the tomtom is booted with the new bootloader you can remove the system file again because the bootloader is written into the eprom chip.
Here is bootloader 5.5128, which is currently suggested for One/XL devices.