As with a computer, a GPS Tech might be able to salvage information even though current GPS will not work. It does have a memory and a tech might be able to help. Maybe you have already tried this, but if not it might be worth a call. The with the manufacturing company, also.
TOM (snookman47) www.TheFishermansCatch.com
(We sell Fishing Books, Tackle, Clothing, Guides Books, Fishing Magazines, Accessories)