Help! My External HDD SD Card Seems to Have Shrunk in Size

Hey everyone! My dad got this "3TB" external HDD some time ago, but it turned out to be only 320GB after I checked. He returned it and gave it to me. I tried to use Balena Etcher to flash Linux onto it for my old secondary laptop, but I ended up not using it. Now I want to format it for storage, but it only shows up as 7GB! I followed a YouTube tutorial that said to use the command prompt and enter diskpart, then clean the drive, but after that, it said the drive wasn't installed. Someone in the comments suggested using Device Manager to reinitialize it. Unfortunately, after all that, it doesn't show up in the system anymore. Any advice on what I can do? Thanks!