Let’s encrypt an external USB hard disk drive. Get root already and type lsblk. Then insert the USB key and check its name by typing lsblk again. In my case, it was sdb1, but yours might be different. Get yours right, or bad things can happen to your other devices.
[root@slayer dth]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
...
sdb 8:16 0 1.8T 0 disk
└─sdb1 8:17 0 1.8T 0 part
...