Can't mount a new partition in Redhat Enterprise with LVM

Hi,

I have a RH Enterprise server, and I am adding a new hard disk to it.
The disk is recognized as sdb, and has been partitioned to /dev/sdb1:

mount /dev/sdb1 /mnt

returns sdb1 already mounted or /mnt busy.

Google returns result shows the cause is device mapper, which is
required by LVM. Currently my another disk are managed by LVM, so I
can't disable device mapper module, otherwise I think I can't boot.

So I try to use pvcreate to use LVM to manage the added hard disk, it
still says the device has been exclusively occupied... I am very angry
about it.

dmsetup ls

don't see any info about /dev/sdb.

So anyone has experience to add a new disk in RH Enterprise?

Thanks.
Bin