Adding a 2nd Hard Drive to My XPS8500

I would like to add a second hard drive to the XPS8500 I bought  last week similar (same manufacturer, model, size, etc.) to the primary hard drive that comes in my system.  I just received the e-mail today indicating it has shipped.  The e-mail contained the service tag number, so I was able to pull up the system configuration and see the primary hard drive that came in it (besides the 256 GB SSD).  My first question is with the description itself.  The part number is 2WK2D.  The description is:  "HARD DRIVE..., 2 TB, S3, 7.2K, SGT-MUSKIE, 6G??".  First, does S3 mean SATA3?  Second, what is a SGT-MUSKIE hard drive.  I Googled it and found several articles that make me believe this is a Seagate drive.  However, when I go to Seagate's website, I couldn't find anything with "MUSKIE".  If it is a Seagate drive, is it one of their regular HDD drives or is it one of Seagate's enterprise drives.  My next question is does anyone know why there are two question marks at the end of the description?  Does it indicate this may or may not be the hard drive in the system or does it indicate there is a question regarding the 6Gb/s SATA transfer rate.  My next question is what you would recommend for the second hard drive.  If the primary drive is a Seagate HDD SATA 3 drive, I was looking at the ST2000DM001 HDD drive (2TB, 64MB cache, 7200, 6Gb/s) drive to be consistent with the primary drive.  I think this drive is SATA 3  and my other 3 SATA drive connectors are SATA 2 but isn't SATA 3 backward compatible such that I can plug this SATA3 drive into a SATA2 drive connector?  I realize the SATA 2 drive connectors are only 3Gb/s but if my primary drive ever goes out, I will plug this secondary drive into my primary drive.  Also, if the "MUSKIE" hard drive is a Seagate Enterprise drive, is there any reason to buy another enterprise drive.  Can someone tell me the differences?