Which is the best way do delete a file on FAT32 file system securely (i.e. make it impossible to recover the file). Is overwriting the file with garbage and then deleting it secure? Could anyone suggest a good reading on this?
You have to delete the file from the file system, and then scrub the unused, freed-up sectors.
Contrary to popular belief, simply writing the sectors with zeroes and ones once or twice will clean them beyond the capability of the vast majority of data recovery companies to retrieve the bits.
Securely Delete a File using .NET
http://www.codeproject.com/KB/cs/SharpWipe.aspx
Secure File Shredder
http://www.codeproject.com/KB/files/NShred.aspx
Overwriting in place normally works on FAT32, but you may need to watch out for intelligent media defeating you. Wear leveling on flash media and block remapping on hard drives can result in a new block being allocated to replace the old one, leaving the previous data in place.
If you trust the media not to screw you over with block remapping, then see the media sanitizing standards specified in DoD 5220.22-M.