4TB trying to erase on mac

i have a iMac with a 4tb 3.5 hdd I'm trying to format it from disk utilities when i go to disk utilities and select the drive its only giving me first aid and partition option when i go to partition option its selected with basically doesn't allow me to change anything can you please help what do i need to do i have a bootcamp already set up but its on a different internal main hdd

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