Welcome to Tom's Hardware & Happy New Year, @sharathkbc007!
I'd strongly recommend you make sure you have a copy of the files you have already stored on the HDD.
It sounds like there could be something wrong with the drive but further troubleshooting is necessary to determine that. I'd advise you to check how the HDD would appear if you connect it to another SATA port on the motherboard and also using a different SATA data cable. If you have access to another working PC, I'd advise you to check how the hard drive appears there as well. I'd also recommend you
run the HDD's brand-specific diagnostic utility and determine its health and SMART status. If the HDD is failing (not passing the Short & Long tests), then I'd advise you to get in touch with a professional data recovery company for assistance! If the only copy of the 500 GB of files is on that drive a professional would be your best bet on getting any of them back. Doing data retrieving attempts by yourself is potentially dangerous to the files, especially if the HDD is physically corrupted.
Hope this helps you. Keep me posted with the troubleshooting!
SuperSoph_WD