Don't let anyone tell you it can't be done. It might not be a good idea, but it is possible. You'll have to add the last two drives via the command line,
and nothing bad will happen after 24 hours. Here's an example: twelve one 1TB SATA drives in an FAS2020 with DataONTAP 7.3.3 nets you 7449GB usable space:
BabyNetApp> vol add aggr0 -d 0c.00.11
BabyNetApp> vol status -r
Aggregate aggr0 (online, raid_dp) (block checksums)
Plex /aggr0/plex0 (online, normal, active)
RAID group /aggr0/plex0/rg0 (scrubbing 24% completed)
RAID Disk Device HA SHELF BAY CHAN Pool Type RPM Used (MB/blks) Phys (MB/blks)
--------- ------ ------------- ---- ---- ---- ----- -------------- --------------
dparity 0c.00.1 0c 0 1 SA:B - SATA 7200 847555/1735794176 847884/1736466816
parity 0c.00.4 0c 0 4 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.2 0c 0 2 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.3 0c 0 3 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.5 0c 0 5 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.10 0c 0 10 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.0 0c 0 0 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.6 0c 0 6 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.7 0c 0 7 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.8 0c 0 8 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.11 0c 0 11 SA:B - SATA 7200 847555/1735794176 847884/1736466816
data 0c.00.9 0c 0 9 SA:B - SATA 7200 847555/1735794176 847884/1736466816
You'll need to live with the "sam.misconfig.noEligibleSpares" warning, though you can make it less annoying by setting "raid.min_spare_count". Assuming scrubbing is enabled and active, you'll need three drive failures before data is lost. Keeping a spare drive in your server closet, while not practical, would be a good idea.
BabyNetApp> aggr show_space -g
Aggregate 'aggr0'
Total space WAFL reserve Snap reserve Usable space BSR NVLOG A-SIS
8276GB 827GB 372GB 7076GB 0GB 0GB