I have a windows app that only needs to scan for the file meta data (File Name, Date Created, Directory, File Size etc) to support my clients. My application FindMyFile.exe builds a virtual drive to store a relational database of that data. I then can query for any file and get an answer in 1-3 seconds (my test database was 2.4 million records).
Many of my clients use DropBox and I want to permit them to include their files on DropBox along with their other file storage medium and give them a complete picture of their data (c:/ d:/ e:/ f:/ and DropBox) to scan for their files.
This app is close to release, and it has a "patent pending" status at this point.