I follow the instruction here http://www.nslu2-linux.org/wiki/Main/PlugComputers

I added a line in /etc/init.d/rcS  "mount /dev/sda1 /opt " before the hostname

but after I reboot

then mount show

~ # mount
rootfs on / type rootfs (rw)
ubi0:rootfs on / type ubifs (rw,relatime)
none on /proc type proc (rw,relatime)
none on /sys type sysfs (rw,relatime)
none on /dev/pts type devpts (rw,relatime,mode=600)
none on /tmp type tmpfs (rw,relatime)
none on /proc/bus/usb type usbfs (rw,relatime)
/tmp/.cemnt/sda1 on /tmp/.cemnt/mnt_sda1 type ext2 (rw,nosuid,nodev,noexec,noatime,errors=continue)
~ #

but if I type mount /dev/sda1 /opt it works

other thing my roots why mount in rw mode?

Thanks