replace defective hard drive in macbook

I have a macbook 2 years old it suddenly gave me the grey screen with question mark. So I work a lot with PC laptops so i replaced the harddrive with a clean laptop drive. I even erased it in disc utility on my macbook pro. When installing the OS it does not find the harddrive. Is it possible im doing something wrong, or is the sata port possibly the problem. I tried installing the OS with the hard drive connected USB ( i have a SATA to USB cord) it would see the hard drive but wont let me install. Thanks for your time.

you need to format the drive first for Mac HFS+

When you get to the install splash screen, you can access disk utility from the top menu (under utilities) and format the drive from there.

If all you were just getting a grey ? screen from the old drive, chances are the old one is OK and just needs a reformat and/or install

OK I am in disk utility it doesnt see it. It does see the hard drive when a different one is connected USB...what is HFS+???

HFS+ is just the format the drive has to be in. For Windows HDD need to be NTFS, but anyway if you can't use disk utility to format it try using a free program called Gparted but for this program you need a windows machine to run it since it boots off DOS and from there you can format it to any format you want. Then it should be able to recognize, if not could be another problem with your mac. Hope this helps. Good Luck!!!