Different things to try before you throw away your corrupt HDD. Mine wasn't mounting, ejecting, etc. I couldn't carry out first aid, erase, or anything else.

BEFORE USING THESE TERMINAL CODES PLEASE WATCH VIDEO FIRST

diskutil list = list of disks in use
clear = clear terminal screen
diskutil unmountDisk force (enter disk name from top code output) = force unmount
sudo lsof|grep (enter disk name from top code output) = shows processes running from disk

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