I had a 20 gb hidden drive, which brought my 500gb C drive down to 465gb. But after deleting the hidden drive, my C drive still shows up as having only 465GB. How do I fix this?
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This is the difference between two different notions of what a gigabyte is. It can get pretty confusing, so let me try to explain.
One definition of gigabyte is "one billion bytes" (i.e., 109 bytes). This is the definition used by hard drive manufacturers. Thus, when your vendor says your hard disk is 500 GB, they mean it has capacity for 500 billion bytes.
Another definition of gigabyte is "230 bytes" (i.e., 1024*1024*1024 bytes). This definition is convenient for computer science, because it fits in well with the binary arithmetic used by computers and used by memory addressing. This is also the definition used by RAM manufacturers and -- of most relevance to your case -- by a lot of software that reports file and hard disk sizes. Note that 230 bytes is 1073741824 bytes, about 7% more than the definition above. Sometimes people use the abbreviation "GiB" for this notion, to try to avoid the ambiguity.
Now, 500 GB happens to be equal to about 465 GiB, since 500 / 1.07 = 465.
So, there is probably no hidden drive and no hidden space on your hard drive. Instead, you are just seeing the effects of a disagreement between different people about what a gigabyte should mean: your hard drive manufacturer is reporting capacity in the former notion (GB), whereas your operating system is reporting capacity in the latter notion (GiB). Whee! Aren't computers fun?
To merge the free partition with your existing partition, to get the full disk available in one partition, you need to use a standalone tool that can run when your OS (windows?) is not running. A great utility to do this, is GParted, which can be booted from a CD/DVD/USB stick and gives a visual partition editor that lets you resize the existing partition over the empty one, by just dragging the boundary.