You haven't said how large you knew drive (sdc) is? Is it 500GBs or
1000? If for example it was 1000, you would need to buy another 1000GB
drive, and set up a second mirror with the two new drives. So now you
have two 1TB mirrors and stripe across them
(http://en.wikipedia.org/wiki/Nested_RAID_levels#RAID_10_.28RAID_1.2B0.29)
giving you 2TB of space that is mirrored also for redundancy (then you
would have the redundancy of mirroring plus the speed of striping, or
near there).