I recently purchased a new PC, and I have my old PC's hard drive that I'd like to put in my new PC. Here's the problem, It still has ALL of my old PC files on it, and If I was to put it in my new PC, wouldn't it mess it up?
You should be able to access your files when you connect your old SATA HDD to the new PC. You should have enough available SATA ports on your motherboard. If you plan to use that drive as secondary storage on your new rig, you should consider backing up the files on your other drive (or an external) and re-formatting the drive (especially if it has your old OS installed or software apps). Once you've copied your data elsewhere successfully and erased the drive to start fresh with that secondary storage, you can simply paste back your files to it.
You won't mess up anything, your new computer with its OS won't be able to recognize the one from your old PC. Don't worry.
Keep me posted if you have more questions.
SuperSoph_WD