http://www.easeus.com/
I had to use the PAID version, but I tried many programs and only the EASEUS Data Recovery Wizard Pro worked, but I think the free version will at least tell you if it's possible (probably a limit on number of files or space recoverable).
Here's what I did.
1. Hookup only two drives (defective one and one to copy data to).
I prefer to UNHOOK all other drives (both cables at rear of drive).
2. Boot to the EASEUS disc (you'd create that first).
You may also run from Windows if that's an option in which case leave all drives attached.
3. Scan defective drive (many hours. My 2TB took 12 hours or so)
4. Select anything found and COPY to another drive.
*So basically, your drive must be functional enough to still work but it's dying or you accidentally formatted it and need to recover the data. This data is then found as individual files since there's no proper index. You then copy to another drive.
Once completed (if possible) RMA the drive if still under Warranty and consider a redundant backup solution in the future (RAID1 or software like Syncbackse Free to auto backup a folder to another drive.)