Since the new hard drive is being returned to the dealer due to a fault, I called the following on the hard drive for secure deletion:
hdparm --yes-i-know-what-i-am-doing --sanitize-overwrite hex:11111111 /dev/sdx
Yes, I didn't know what I have done!

The operation will take 10 days.
If the hard drive is plugged back in after powering off, it will continue working on its own. What can I do other than wait? Hope it gets done before the return deadline.