mount_null: Operation not permitted

[Siju George]

Hi,

I have been trying to

dfly-bkpsrv# mount_null /dev /mnt/2ndDisk/dev

So that I can update the system on my second disk.
I was able to mount /dev on /mnt/2ndDisk/dev successfully till a few days back.
But now I get this error.

dfly-bkpsrv# mount_null /dev /mnt/2ndDisk/dev
mount_null: Operation not permitted

What could be wrong?

Thanks

Siju