BUT here's the good news: you did NOT lose any space! When a disk manufacturer says the unit holds "500 GB", they really mean 500,000,000,000 bytes. But Windows uses "GB" to mean 1024 x 1024 x 1024 bytes, or 1,073,741,824 bytes. Using that definition, it calls that same 500,000,000,000 bytes "465.66 GB".
Now, what about the space used up by files? Well, Windows still uses that definition of "GB". So if it says your big file is 1.000 GB, it is actually 1 073 741 824 bytes! In this system, by the way, 1.000 MB is actually 1024 x 1024 bytes, or 1,048,576 bytes. Looked at another way, to store a file that actually has 1,048,576 bytes of data in it, it will occupy on your drive a space that Windows will tell you is 1.000 MB, NOT 1.049 MB. The space is still all there. Windows just uses a slightly odd-sized "yardstick" when it goes measuring MB and GB.