"I am trying to integrate a block device driver. The driver works the first time, it gets installed and then I am able to mount and do all the operations on the card.
The problem is that after I umount the card, and then try to mount it, says unable to find Fat 0 Sector.
Bogus File system found."
I would try the same test with the normal vfat driver that comes with Linux and see if you get the same error. If you don't then I would say that there is a bug in the block device driver that you are trying to integrate.
Be prepared. Create a LifeBoat CD.