When you connect a new drive, if it contains any RAID volumes, mdadm automatically recognizes them and assembles arrays (but fortunately designates them inactive/read-only until they're accessed). This is unhelpful when you're just testing or wiping drives, because it marks the drive as being in use by the kernel before you've even done anything with it. To prevent this behaviour, you need to negate /lib/udev/rules.d/64-md-raid-assembly.conf (As a one-off you can just delete this file, but in general you'll need to write a rules file in /etc/ to negate its behaviour, because files in /lib/ will come back when udev is updated.)