Data Recovery Druid 4+

Note: Although the app store version is limited, the website version is fully functioning and also capable of scanning internal disks.
If you purchase the App Store version, the website version will be automatically registered as soon as you download it and you can use that to scan and recover data on any disk.
After using the In-App Purchase in Data Recovery Druid from the app store, simply go to the website and download it from there. It will automatically detect that you completed the in app purchase with the app store version, and will automatically register itself so you can use it to scan your startup disk.

*** Alternatively, if you only want to use the App Store version ***

The App Store version of the award-winning data recovery technology, uses uncompressed (byte-by-byte) disk images as the source for data recovery. Here's how you can create them simply:

1. Run Disk Utility, it's a standard Mac OS X app.
2. Go to File > New > Disk Image from "YOUR_DISK_NAME".
3. Choose "read-write" as your Image format. Do not choose "Compressed", or lost data will not be copied.
4. Set encryption to "none".
5. Save your disk image, and load it in Data Recovery Guru when done.

From AppleLinks
“I found that Data Recovery Druid works as effortlessly and elegantly as claimed, and that there is only the most minimal learning curve involved. Using it is simple and intuitive. It does indeed “just work”."

In a similar manner as the Finder allows you to browse through regular files on your disks, Data Recovery Druid allows you to browse and recover deleted files.

Although the App Store version is limited to scanning external devices (due to App Store security policy), if you purchase the App Store version then download it from the website, the website version will automatically be fully registered and capable of scanning your startup disk.

With this free version you can recover 10 common filetypes, and preview the less common types that are available for recovery. The common types can be recovered with the initial purchase. There is an In App Purchase for the ability to recover the less common types.

It allows you to view files that were in the trash after it's been emptied, and files from formatted, corrupted, or unmountable disks or other devices such as cameras and phones. You'll be surprised at what it'll find. It can often show you files you would never have thought were still on your disk, deleted years ago.

In the same way you browse through your regular files using the Finder, Data Recovery Druid will scan your disks for permanently deleted files and allow you to view them in a media rich file browser window.

Notes:
• If you purchase it with the Mac App Store and you then use the website version, the website version will be automatically fully registered. You can seamlessly use it to scan the startup disk.

More Reviews:

From Tech-Reviews
“Unlike other data recovery applications, this really is a very simple application to use and does exactly what it says on the tin.”

From MacDetails
“We recovered everything we could find – Word docs and PDFs, Illustrator files, QuickTime movies, and Photoshop PSD files.”

From MacManiaPodcast
“allowed me to quickly and easily browse them with the preview. Then, I was able to click the “Recover” button and save them right to my desktop. It worked perfectly, and salvaged the files that I needed.”