Recover files from unallocated space External hard drive [Solved] - Western Digital - Storage
How Can I Recover My Data from an Unallocated Hard Disk Space?
These sites seem to have the most relevant information/guides you're looking for.
Since this is playing around with your data, I should tell you that I only googled the issue and read the sites over. When it comes to recovering files, it's been hit or miss for me. I've never dealt with this particular problem.
Good luck!