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

To prevent this behaviour, you need to negate
(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.)