Hi,
Thank you for posting your query in Microsoft Community.
I understand that 500GB space on your 1TB HDD is missing even after formatting it. Let's try the following methods and check.
Method 1:
****Step 1: Check if the issue occur in safe mode:
Please refer to: Start your computer in safe mode
Step 2: If the issue doesn't occur in safe mode then you may perform a clean boot and check.
How to perform a clean boot to troubleshoot a problem in Windows Vista, Windows 7, or Windows 8
Please note: After the troubleshooting, make sure to boot your computer to normal mode by following the step 3.
**Method 2:**This issue might be caused due to any malware in the system, so you may run a Microsoft safety scanner to make sure computer is free from virus infection:
http://www.microsoft.com/security/scanner/en-us/default.aspx
**Disclaimer:**If you run the virus scan the program that is infected with virus will get deleted. So you have to reinstall the program. Also if the files and folders are affected with virus then they might even get deleted
Hope this information is helpful. Please feel free to reply in case you face any other issues with Windows in future.