To my fellow posters: if you don't know the answer, please keep your hands off the keyboard! Ill-considered, *untested* answers that send people down deadends are not "helpful".
Glenn, Remy offered one of two likely reasons for this (LVM engine already open).
The other is that your 2nd disk isn't using an MBR layout. To keep LVM engine from destroying disks laid-out using GPT & LUKS, a crude fix was added: if it finds one of these disks, it won't even start the LVM engine. The solution is either recreate/repartition as MBR or just blank the first 63 sectors of the disk. If this still doesn't fix the issue, I can PM you a workaround to get rid of the message (AYOR!).
FWIW... in AOS 5.1, GPT disks will be supported and any other type of layout will be hidden by the disk drivers, so this problem shouldn't arise.