when I "ls /dev/dsk/ ", I can see c0t0d0 and c0t2d0;
but when I use format command, I can only see c0t0d0 available;
why this happens? how many disk do I have on the machine? (I do not know that)
You can start by trying a few things.
Try running prtdiag and see what info you get from that.
Also give the server a physical inspection if possible to see if you do have 1 or 2 disks.
run a 'df -k' and see all the disk devices being used and see if you can spot anything.
If this was a new disk added, then it might need a devfsadm to be noticed by the OS.
These steps should lead to some clues.
-S
try a "devfsadm -C" and look again to /dev/dsk. if c0t2 is gone, the drive is maybe removed or broken...
afterwards you can try the "devfsadm" or a "reboot -- -r" to look for new disks.
hth