Tomtom 520 reduced memory

Hi
Can anyone please help ?
I have lost my maps on my 520 and the internal memory size is now only 485 mb !
I made a backup but due to memory size cannot now put back on.
Please help

Do a checkdisk of the drive.
If there are error let checkdisk correct them.
Delete everything on the device and put your backup on again.
In case checkdisk can't give you back the full size you can try a quickformat with standard settings in Fat32.
If still no luck your memory might be faulty.