2012 MacMini HDD died while installing Win 10 Tech Preview
I'm not 100% sure this will work in your case since I think your partition map got erased (or something similar) but try this TestDisk tutorial.

I also used to know the site of a really nice walkthrough that helped me fix something similar on a flash drive a little while ago, but unfortunately the tutorial was only for Windows. Here's a site I found with some similar information for both Linux and Windows. You might be able to get this to work for you if you can run Linux off a flash drive or something.

Since you mentioned TestDisk, here's another guide for fixing the partition table of your disk using TestDisk. Again, this article is for Linux, but I assume TestDisk will work the same on both operating systems.

If it works, the first link will give you the cleanest, most-direct fix.

Using the last two links, you should still be able to fix the disk by first formatting the partition table to some universal format like FAT and then running your OSX recovery program to reformat the disk as it sees fit.

If you do go with the last two links, I would recommend trying to run PhotoRec on it first just in case you're able to recover any useful data with it. PhotoRec is made by the same folks who made TestDisk, but it's easier to use and is designed to just bruit force scan the disk and automatically pick out files that it can recover — actually "bruit force" might not be the right word here since that's just one of the options, but you get my point. :)

Good luck!