Boot the ZorinOS boot USB. In the Disks application, remove all the partitions on all of the drives that will be going with the computer when you sell it.
Then zero all the sectors on the drives:
sudo dd if=/dev/zero of=/dev/sda bs=512 status=progress
You'll have to change the /dev/sda blurb to match whichever drive you're zero'ing.
If you want to do more than one drive at once, open more than one terminal window and use the command above, one for each drive. If the drives are on the same interface, that'll slow the data transfer to each drive, if they're on different interfaces, you'll get them all done faster than doing them serially.
The password protection for the drives is likely in the BIOS / UEFI... it'd probably be best to delete that password and allow drive access without password protection before you sell the machine, so the new owner isn't locked out.