All most any "FREE" anti-virus software has a ton of popup ads and or catches, like it will scan it but until you 'purchase' the FREE software they will not remove the threat.
Think about what you are trying to protect (your server) -- for a fraction of the price of the server you can purchase a good anti-virus like Norton, McAfee, or Kaspersky, But if you really want a FREE one AVG is a fair product.
Personally I would scan the removable drives with another computer or laptop THEN connect the uninfected drive to the Blackstorm