what is the udev rule to allow specific vendor thumb drive?

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http://stackoverflow.com – I need a udev rule so that it should detect the vendor of the USB pendrive, and allow only specific vendors, rejecting all others. How to write such a rule ? Where should the rules be placed /etc/udev/rules.d or /lib/udev/rules.d/ What is the command to get effect of the udev rule. urgent plz (HowTos)