It not only has to be not 'open' in the explorer, but properly unmounted. Tools like mkfs dont do that for you, its just not their job. (and might be unwanted or stop your from making mistakes like accidentally overwriting the wrong drive)
try umount /dev/USBDRIVE
If that still complaints about Device or ressource busy, then something is still using it.
Either try to close things that might be the culprit, reboot and try again or, if installed and you are compfortable, you can check which processes using lsof -D <path where drive is mounted to> (you can get that location using mount | grep <path to usb drive>)