As far as I know, you cannot only "Mirror" a partition.
As Todda stated, you need a pair of the same drives. When you create the "mirrored" array, you can then partition the drive that is mirrored as you wish. All partitions will be mirrored in the array. At least, this is the way I understand it. I run RAID 0 which is not true RAID since there is no redundancy.
Mirroring is really only for safety, to protect against data loss in the event of a hard drive failure. In that respect, it is truly useful. But for your need, it won't work.
Main DVD MM Rig:
CoolerMaster Storm Scout
AM3 Phenom II x4 955BE @ 3.6 ghz
Corsair H50 Watercooling kit
4.0 GB AData DDR3
GIGABYTE GA-MA770T-UD3P Mainboard
Western Digital 750GB 7200rpm w/32mb cache
Samsung 750GB 7200rpm w/32mb cache
Western Digital 1.0TB 7200rpm w/32mb cache
LG 20x DVDRW-DL "SuperDrive"
PNY GeForce 9800gt 1024mb w/Samsung 245BW 24" LCD
Corsair 620w Modular Power Supply
Logitech z-640 5.1's
Logitech Wireless Mouse
Win 7 Ultimate........