@machinchose
DS 40 and DS 50 use the same firmware but DS 51 is a completely separate device with it's own firmware. Also afaik the latest fw for DS 40/50 is V5.90, there is no V6.10 for that series yet, however DS 51 had previous version 6.10 and is now at V620.

If your final paragraph means that your DS 51 is in a bootloop then to unbrick look here: [Only registered and activated users can see links. ] (not just for nuvi models) and here: [Only registered and activated users can see links. ] and if you need more help finally here: [Only registered and activated users can see links. ].