External hard drive won't mount

When I run disk utility and try to repair my hard drive I get a error exit code 8 and disk cannot be repaired. My hard drive shows up in disk utility but it can't be mounted. I downloaded some recovery software and it can pull up all my files (thankfully they are still there) im trying not to pay the $50 to actually buy the program so I can actually recover them.The program will only recover 100mb at a time for free and a lot of files I have are larger than that. I tried it on some smaller files and it works. So my main question is how can the recovery software search my drive and find the files if the hard drive isn't mounted. And my second question if this program can do it while the drive is unminted isn't there a way I can do it manually so I don't have to spend $50 for the program to do it for me?