add HD to help /usr?

[perikillo]

   Hi all, i have freebsd 5.4 release on one small HD, but i have
another one that i can use to help /usr went i install ports, upgrade
the system or make new kernels, but i dont know how i can setup this,
i already config the hd went the the name i give to the slice is
/usr2, i have this on /etc/fstab:

/dev/ad01s1d      /mnt    ufs  ro,noatime    2    2

   Them how i can use this slice to help /usr to buildworld, build
kernels, and build ports, any information i will apreciate, thanks to
all.