Only half of 8TB Available

I have an 8TB Seagate hard drive that was connected to my 4 Tuner Tablo. I used up about half of it. I’m trying reattach it but it only has about 4TB available. I have about a 7 year old laptop that runs Windows 10 but can’t figure out how to format the drive. I don’t care about the old recordings but would like to have the entire drive to record on. I’m hoping to figure out a way to get the Tablo to format it for me. Any suggestions are appreciated.

Right click over the start button and enter disk management. You should be able to format it from there.