Comment 12 for bug 146599

Device ordering is not guaranteed since it can be affected by your hardware controllers taking more time on one boot to warm up than on the other.

When you have multiple controllers like this, swapping drive ordering is quite common.

(Laptop users often see the same when their wired and wireless ethernet cards often swap around).

This is why we use UUID= to mount your disks everywhere, so they're located by a property of the filesystem on the disk rather than a transient name.