Hello.  I have an ASUS Desktop PC Series CM6870.   It has an Intel Core i7-3770 CPU, 16 GB RAM and runs Windows 10 at 64 bits.
My problem concerns the partitioning of my internal hard drive.  The internal drive is 2TB but was partitioned into two by the manufacturer.  The "Windows (C:) drive was only allotted 149GB. The rest constitutes the "D:" drive and is 1.65TB.
Over the course of using the computer, the C partition has become full. I don't download or store anything on that partition. I haven't installed any new software but all my program files are on the C drive.  The amount of remaining space on that partition seems to fluctuate several GB per day but generally gets smaller.  I have done what i can to try to locate what is taking up all that space. the C>Windows folder takes up about 55GB of space.   The C>Users folder takes up about 25GB.
I tried to increase the size of the C partition. I used Partition magic to create a new partition from the D partition. I can't seem to now merge the new partition with the C partition.
I would appreciate any help with this problem.
ThanksAFAIK you can't do it with built in Disk Management (WIN+X, click Disk Management). All you can do is the standard way, resize C: so it can contain everything from D:, move everything from D: to C:, delete partition D:, resize partion C: to include the unallocated space.

However it looks like there's a free tool that can merge two partitions "magically" although it's not partition magic. I haven't tried this so please do so at your own risk, but here's the doc on the software website

https://www.easeus.com/partition-manager-software/combine-partitions-windows-10-free.html

best of luckyou could RELOCATE things from the C drive to the D drive, like User files and pagefile.

User files, like Docs, Pics, Music etc you right click on and hit Properties > LocationSee if this is helpful. Let me know if any questions

https://www.easeus.com/partition-master/partition-magic-windows-10.htmlHI
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I tried to increase the size of the C partition. I used Partition magic to create a new partition from the D partition. I can't seem to now merge the new partition with the C partition.
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When you want to increase the size of a partition it needs to have free space. It seems you have created another partition where you should have deleted the D partition and left it un allocated and then resized the C Partition including the free space. Windows Disk manager or any of the tools others have suggested can do a resize . It is always best to Backup the drive first then if anything goes wrong you can restore the backup making the C partition larger. 
If he deleted the D: partition all his data is gone...not a good choice.

He needed to create a new partition from free space on D:...then merge it with C:...
From dictation.
Here are some general observations about using tools such as the EASUS PartitionMagic or similar tools for altering partition size on a Windows based system.
First of all, the user should have a backup plan already in place before making any attempts to change partition size or the number partitions.
Next, the disc drive should be diagnosed with at least the check disk utility that comes with in Windows. If the check disk utility reports errors on the drive, any attempts to partition the drive may result in errors.
An option is to also try using the defragmentation utility. It might not be necessary, but it might make it easier for the disk partitioning software to resize or move any partition.
If the partition to be altered or removed is also the same partition used to boot and operate the operating system, it might be necessary for the partitioning software to reboot the computer and start the partitioning task before the operating system is loaded.
Based on my own EXPERIENCE here are some problems I think might be relevant.
Every time something happens to increase the possible drive space that can be allowed on a disk drive, that turns out to be some kind of hardware or software problem that interferes with larger disk size. Even with the newest methods in use, there still is a problem when you get to extremely large drives. Specifically, over 2Tb there is a BIOS issue taht can not be fixed with just the any disk partitioning software.
Windows 10 and a few programs do not really take up a lot of space. The space REQUIRED is usually less than 100 GB. If something is using up an awful lot of space for no apparent REASON, it's possible there may be some kind of mall where present on the system that is trying to propagate itself. But I don't know how to prove that. Nevertheless, using some type of virus scanner should be a frequent practice of any user that ever gets on the Internet.
In the worst say case scenario, you have to erase everything on the huge drive and then partition that as desired and after the partitioning is done install the operating system from media such as a DVD or a USB device with Windows 10 installation software.
Trying to move or repartition of very large drives is very time-consuming. Especially if the drive is near full. In fact, if the drive is very close to being full in may be near impossible to change the partition size at all. There has to be some wiggle room for they partitioning software to move files around. If there is a very large file that needs to be new move and there isavailable for that large file, the partitioning software has to figure out some other way to make adjustments to the partition. In some cases it may not be able to complete the task because there is one or more files that are just too large to move.
An empty drive is much, much easier to partition. The software does not really have to move any data around, just change the size of an empty partition. He can actually take less time to do a backup, erase the drive completely, partition the drive, and copy the operating system and other software back onto the newly partitions Drive.
The maker of the drive partition software should have a knowledge base that deals with issues involving very large drives and partitions that are nearly full.

I hope this is good is of some help.   
 Microphone off

EDIT: Here is a link taht might be relevant:
https://support.microsoft.com/en-gb/help/2581408/windows-support-for-hard-disks-that-are-larger-than-2-tb
Quote

In order for an operating system to fully support storage devices that have capacities that exceed 2 terabytes (2 TB, or 2 trillion bytes), the device must be initialized by using the GUID partition table (GPT) partitioning scheme. This scheme supports addressing of the full range of storage capacity. If the user intends to start the computer from one of these large disks, the system’s base firmware interface must use the Unified Extensible Firmware Interface (UEFI) and not BIOS.

I'd suggest making a second partition but not merging it with the C Drive. The more that gets put on a partition the slower it becomes. I have different partitions for music, videos, programs, phots etc.