External HDD (Internal) Not Showing Up In Finder, But It Is Showing Up In Disk Utility

Before I start, I know that this question has technically been solved before, but not exactly in my case. Keep in mind that I'm a graphic designer, this is why I am so passionate about fixing this issue. So, here's the story. Yesterday, I sold my Mac mini (Late 2009). I had plans of buying the new MacBook Pro w/ Retina Display (15 inch), and that worked out perfectly. I bought the MacBook and sold my Mac mini. Prior to selling the Mac mini, I decided to format an old hard drive that I had, which was a 160GB hard drive. My plan was to sell the computer with the 160GB HDD, and keep my 750GB HDD. I know my fair share of computing and repair, so I replaced the hard drive myself, and reinstalled Mac OS X ML on the 160GB HDD. I installed the OS via external SATA to USB connection, and it booted up perfectly through that connection to my Mac mini. Then, when I took out the 160GB HDD, my computer booted to my priority internal drive, which was the 750 GB HDD, that I'm having problems with currently. It booted up perfectly, and all of my information was on it. So, at that point, seeing that everything was flawless, I extracted the 750GB HDD, from inside the computer and replaced it with the 160GB HDD, which I formatted, reinstalled ML on, and was planning on selling it with. The 160GB HDD booted perfectly and everything was working on the Mac mini, but then, I plugged in my 750GB HDD via USB connection, to see if my files and everything was still there, I got a message saying, "The disk you inserted is not readable by this computer" and it gave me three options underneath, which were: "Initialize...", "Ignore" and "Eject". I pressed on initialize, and tried to do everything that I possibly could, without formatting it, and all of the options like "Verify Disk" and "Repair Disk" were "grayed out", if you know what I mean. I tried plugging it into all 5 USB ports, but I got the same message. It was recognized in Disk Utility, but not in Finder. So, I sold the computer with the 160GB HDD that was working fine, but now I had another dilemma which was why my 750GB HDD, isn't showing up in Finder. I can hear the HDD spinning when I plug it in via USB. This Hard Drive has run Mac OS X Lion and Mountain Lion on it, I've never installed Windows on it. So, it was properly formatted for a Mac. Now, I went out and bought the MacBook Pro with Retina Display that I needed, and I tried doing the same thing, which was plugging it in via USB, and seeing if Finder would pick it up. Still, nothing. I ended up with the same message. Here are some reference screenshots, of what I'm getting exactly.

I've called Western Digital, and they told me to try booting up the 750GB HDD, in an older Macbook Pro, to see if it will boot, and recovery all of the files from there. The problem is, I don't know anybody with an older MBP. I called Best Buy and they said that they could take a look at it, but I would truly prefer by fixing this on my own, without having to pay them. That would be my LAST resort. I'm aware that I can't boot up the 750GB HDD in my new MBP w/ Retina because it can only carry SSD. So, I'm basically puzzled. If anybody could help me out, it'd be greatly appreciated. Like I said before, I'm a graphic designer, and I'm in desperate need of the files that were on my 750GB HDD. Thanks.