Today I got two new hard discs for my local file server (thanks to my mate
Tommy for the discs

). So I quickly mounted them into the case of my server and booted the box.
As expected the discs were dected as well by the BIOS as by SunOS. So all I had to do, was adding them to my current ZFS data pool- which was done in less than two minutes.
Verify the OS detected the discs:
QUOTE:
AVAILABLE DISK SELECTIONS:
0. c1d0 <DEFAULT cyl 15014 alt 2 hd 255 sec 63>
/pci@0,0/pci-ide@1f,1/ide@0/cmdk@0,0
1. c1d1 <drive type unknown>
/pci@0,0/pci-ide@1f,1/ide@0/cmdk@1,0
2. c2d0 <DEFAULT cyl 15014 alt 2 hd 255 sec 63>
/pci@0,0/pci-ide@1f,1/ide@1/cmdk@0,0
3. c2d1 <drive type unknown>
/pci@0,0/pci-ide@1f,1/ide@1/cmdk@1,0
There we are... c1d1 and c2d1 are the new discs (currently w/o label, that's why the drive type is unknow)
Check the pool status:
QUOTE:
# zpool status
pool: data
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
data ONLINE 0 0 0
mirror ONLINE 0 0 0
c1d0s5 ONLINE 0 0 0
c2d0s5 ONLINE 0 0 0
mirror ONLINE 0 0 0
c1d0s4 ONLINE 0 0 0
c2d0s4 ONLINE 0 0 0
errors: No known data errors
# zpool list
NAME SIZE USED AVAIL CAP HEALTH ALTROOT
data 76.8G 26.3G 50.5G 34% ONLINE -
Everything's fine- as expected

Add the new discs to the data pool:
QUOTE:
# zpool add data mirror c1d1 c2d1
Done! Now have a quick look, if everything has been added and that's it!
QUOTE:
# zpool status
pool: data
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
data ONLINE 0 0 0
mirror ONLINE 0 0 0
c1d0s5 ONLINE 0 0 0
c2d0s5 ONLINE 0 0 0
mirror ONLINE 0 0 0
c1d0s4 ONLINE 0 0 0
c2d0s4 ONLINE 0 0 0
mirror ONLINE 0 0 0
c1d1 ONLINE 0 0 0
c2d1 ONLINE 0 0 0
errors: No known data errors
# zpool list
NAME SIZE USED AVAIL CAP HEALTH ALTROOT
data 153G 26.3G 126G 17% ONLINE -
Great! Isn't it?!