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Windows showing different max SSD space then bios

I have inherited an AData 256GB ssd. Bios confirmed this is 256GB. Windows Explorer is showing 499MB of max space (364 KB free). I am wondering how come Windows Explorer is showing only 499MB? How would I access the remaining ~255GB from Windows?

Note that Windows is installed on this ssd. My future plan is to get the Windows product key, somehow do a fresh install of Windows, and use this ssd on another computer.