I got this error when I was working with my one DBA, after so lots of basic finding, I came to know that this error occurred when anyone try to attach a Database (Physical MDF/LDF files) from any folder or copy from elsewhere, was previously attached with different SQL Instance (or same instance but previously startup account was different) and that instance Startup account (SQL Server Service) is not as your current startup account.
To resolve this issue you have to provide full access of physical files containing folder/files to that SQL Server startup account. After providing full access of that specific service account for folders/files, now you are able to attach that DB files.
WikiDBA 