this is a common problem, its caused by corrupt data on the SD card. it happens when the unit is powered down while its saving data. to avoid it you want to shut down using the menu or exit navigation before shutting down.
now, you have 3 options:
Option one: Visit C&C wireless
and pay over $100 for a map update or even send them your unit for even more $. see http://www.finedrive-acc.com/shop.cart?action=ITEM&prod_id=F400-08CP
Option two: The other solution was to send it to Fine Digital
but they closed their doors in the US so they no longer support us customers. or C&C also offers a repair service for about $150.
Option 3 Because Fine Digtal closed their doors and left us all stranged, I made a DVD
with map data I purchased from them before they closed and I added much better instructions; this DVD updates the firmware in the unit and loads the last map data issued to the SD card. you would need a PC
with a DVD drive and SD card reader to use it. you can find these DVDs
for $18 which basically covers my costs only. to find it, search ebay for finedrive dvd...