When I inserting an usb device,I was getting an error notice like "Unable to mount <device name> Unauthorized".

This could be due to USB malfunctioning. Install antimalware on your computer ans scan. PANDA 2013 would be the best option. It has antimalware against every malware.

A virus inside a usb stick doesn't have any effect into a linux machine, because the autostart.inf file is not considered and the eventual malware won't start without user action.

The problem is more related to permissions or settings: https://help.ubuntu.com/community/Mount/USB

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