First Remove the sd from Nuvi when you try to format.
It could be an USB port or cable problem:
I would suggest you to use a different USB port and check if it works.
Use another USB cable.
Don't use USB Hub.
Disable your Antivirus or/and Firewall
I would also suggest you to try with a different computer with XP or 7 if you have Vista.
If nothing helps try
Code:
Please Login or Register to see the links
with "write zeros" option.