How can I mount 2nd Hard disk (IDE)

[Kenwey]

I get a warning message when I boot my FreeBSD 5.1, it shows as follows:

Detecting Primary Master ...WDC WD400JB-00ETA0 (40GB, running FreeBSD 5.1)
Detecting Primary Slave ... None
Detecting Secondary Master .WDC WD400BB-32CXA0 (40GB, /lib can't work well)
Detecting Secondary Slave ASUS CD-S520/A
SMART Failure Predicted on Primary Master: WDC WD400JB-00ETA0
WARNING: Immediately backup your data and replace your hard disk drive
A failure may be imminent.
Press F1 to conitnue, DEL to enter SETUP
02/16/2000-VT694X-P4V4X-00


I turn off the machine, and pull it (WDC WD400JB-00ETA0) from 
master and connect it to slave, then I do the following to backup :

# mount -r /dev/ad2s1a /mnt

it shows: incorrect super block

# disklabel -r ad2

the last line shows "super block 0"

do anyone know how can I repair it? is there any way to backup the data 
from this device?