I currently have a 1TB internal hard drive. I am looking to upgrade it to one that's on sale on Newegg. I was wondering if there is a way to just move everything from my old hard drive onto the new one.
Since you are buying a WD new 2TB unit, go to the WD website and download and install (on your current 1TB HDD) their free utility package Acronis True Image WD Edition. Make sure you get its manual document and read it - this package does a LOT, but your interest is in a function called Cloning. You clone your old drive to the new one (this means copying absolutely everything, making sure the new one is bootable and has all the key things in the right places), adjusting a few things for size. Then you swap - suggestions below.
To do the cloning, you mount your new unit in your machine alongside the old one and boot up, then start the utility. A HINT for you. When I've used these, it offers a default set of configuration choices for you to consider and agree to before starting the job. I think one of the common defaults is wrong, and you will need to NOT agree - change one thing. Often the default setting is to Create on your new HDD a Partition the SAME size as your old HDD. But I'm betting you plan to have your new C: drive be 2 TB, not 1 TB! So you need to use the menus to change that setting before letting it go ahead. This is where reading the manual will help! Other than that, the defaults usually are fine.
AFTER you have completed making the clone, here's what I suggest. Shut down the machine, disconnect power, open it up. Disconnect the data and power connectors to your old HDD. Now disconnect the new HDD's data ribbon from the port you had it on, and connect that new unit to the SAME port the OLD drive was hooked up to. You see, your BIOS is set to boot from that port, so connecting to that port the larger drive that now has your OS on it will just work! Close up the case, boot up, and enjoy! In the meantime, that old HDD sitting inside the case disconnected, is a perfect "backup" of your system right up to the cloning operation.
After you are fully convinced that this has all worked, you can reconnect the old drive (to a different SATA port) and use Disk Management to Delete its old Partition(s). Then Create a new Partition (this one does NOT need to be bootable) and Format it - some systems combine these two into one step called Initializing the drive. Or, you could use it for some other purpose - mount in an external enclosure, for example.
Creating a RAID1 array from a 1 TB and a 2 TB HDD will actually create on the new 2 TB unit a 1 TB Partition and copy all to there. After that, if you Break the array into separate drives, the new unit will appear to be a 1 TB drive! And it will have 1 TB of Unallocated Space. Not what OP wants.
Using free good cloning software to make a clone seems the better way.
Well, you can't actually merge Partitions. What you can do in most cases is Expand an existing Partition to include any Unallocated Space that is immediately AFTER the existing Partition. And that is exactly the structure you would have after following your proposal.
However, there is a problem still. Most versions of Windows will NOT allow you to Expand or otherwise manipulate the Partition that has the OS (Windows) installed on it simply by using the Disk Management tools. This is Windows' way of preventing you from risking damage to the OS drive that might render it useless. Third-party software can do it, though - usually you just have to pay for it, download and install it.
Compare the options we are discussing. One is to install a new HDD twice as large as the existing one, the use the RAID management software in BIOS to create a RAID1 array, thus copying everything from the old HDD to a Partition on the new one. Then you reboot and use the RAID management software again to Break this array into 2 separate drives. Then you pay for, download and install on your old drive third-party software, then use it to Expand the 1 TB Partition on the new drive into a 2 TB Partition. Reboot and check it's all OK.
Alternatively, you download and install on your old drive a free utility package specifically designed to include cloning functions. You install the new HDD in your machine, run that software to make the clone copy (adjusting the clone's Partition size as I suggested), and reboot to check it's all OK. We've just arrived at the same point - ready to swap drives and make the new large one the boot drive called C: . By the way, the reason that HDD makers let you download a free copy of their utility package is that it is just perfect for this job - it makes it easy - and that's an incentive for you to buy a bigger drive from them!