Mac HDD corrupt? reads unformatted?

I have an HDD from my old Mac, that I replaced with an SSD. I figured I would hold on to it if I ever needed any data off of it, and sure enough I do.

so I tried to mount it in my enclosure and in my current MBP but it appears to be empty. I know its not empty unless its corrupt and from what I thought, a corrupt hard drive doesn't usually appear empty.

Are there are free tools I can use to see if the data is recoverable, or has anyone had a similar experience with a hard drive that they knew/thought was good, showing up as unformatted?