I can’t say about Ubuntu, but on my openSUSE 10.3 box there is a scan directory option in the context menu, but I never use it the GUI scanner and I have our differences. :stuck_out_tongue: Take my advice, save yourself time, system resources, and several headaches and use the command line scanner, all you have to do is type in the following commands:
sudo avast-update
sudo avast /(location of where your system mounts to)/(name of your USB drive)

If I were to do it on my computer I would have to type “sudo avast /media/disk” to scan it, because SUSE mounts to /media (others could be /mnt, /mount) (as Tech pointed out), then I have my system mounts my USB disk in a directory called “disk”.