Like Patonb said, there is no guarantee that your data will be secure.
You can do it through disk management. If losing data is your concern, back it up to external media such as a External HDD, flash drive, another computer, etc...., if you can.
Keep in mind that if you want to merge drives in disk management, the drive that you want to merge needs to be TO THE RIGHT of what you want to merge it into.
So for example: If you wanted to merge your D drive into your C drive, Drive C would need to be on the left, and drive D would need to be to the right
immediately after drive C.
If they are not in this format, you may not be able to change the drives to suit you. This is where you can use a program called Partition Wizard.
Partition wizard is easy to use, and allows you to merge drives without the limitations of Disk Management.*
You can download it here:
BEST FREE Partition Manager Software for Windows supports all 32-bit & 64 bit Windows No-server OS.
Hope this helps in some way.
*There may be limitations that I have not encountered yet. Some of the advanced features are not available in the home version, but you probably wont need or use them.
The program is not faulty or a virus either, it works fine. The people who wrote the program might not have been good with english though; I have seen numerous mistakes. Just wanted to point that out, there are many faulty/malicious programs with bad spelling/grammar, but this is not one of them.