N Anti-V says 'repair failed' 'no permission to access" for virus on external backup drive

Hi, Norton Antivirus has been detecting and repairingTrojans fairly regularly on my iMac OS X 10.8.3 lately... I finally decided to scan my Iomega external backup drive and NOrton detected Trojan.ADH.2 x 3 (no permission to access); Downloader.Dromedan x 2 (repair failed), PHP.Backdoor.Trojan x8 (repair failed).  What do I do now?

Thanks!!!