Autonamed db error with WD2008Express

Hi All. Can anyone pls shed some light on this error that I keep on getting with a webapp developed for my work. Thnx
An attempt to attach an auto-named database for file C:\Inetpub\wwwroot\tracker\App_Data\aspnetdb.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share