Xbox hard drives (360, One, Series) use proprietary formatting, firmware, and file systems tied to the console. You cannot plug them into a PC and use them as external storage without reformatting—which destroys their Xbox value.
Search eBay "Sold Listings" for your exact drive model (e.g., "Xbox 360 320GB hard drive") to see real transaction prices. Condition + original casing/caddy matters.
Pro Tip: If the drive is in a working console you're keeping, don't sell the drive—upgrade the console with a larger one instead.