Data Recovery Druid

Data Recovery Druid is a data recovery tool for Mac OS X. The app store version is limited; it allows you to recover up to 10 common file types and preview other less common formats.

The website version, however, is fully functional and also capable of scanning internal disks.

You can use the App Store version and buy the full version from there, then download and install the website version, which will automatically detect that you completed the in app purchase and will automatically register itself so you can use it to scan your startup disk.