Read with care threads with instructions:
To easily get preboot follow with care this thread:
[Only registered and activated users can see links. ]
and for cure follow step by step Kunix's thread linked above.
To format:
[Only registered and activated users can see links. ]

Alternative is to send Nuvi to Garmin and get a refurbished one, probably.
If you need help ask here
Be sure to use for cure the right firmware (SG/MY)
[Only registered and activated users can see links. ]