Avast not scanning exterior disk

[Jewel5]

1. Avast tells me a USB disk has never been scanned, even tho it scanned it a week ago.
2. “Scan now?” Click NOW and nothing happens. Not even 1%.

[Asyn]

• Which Avast…? (Free/Pro/IS/Premium)
• Which version/build of Avast…?
• OS…? (32/64 Bit…? - which SP/Build…?)
• Other security related software installed…?
• Which AV(s) did you use before Avast…?