You can have a main drive with your os on it only and not part of the raid. Then use the other drives as your raid, with raid1 the hd's have to be the same size and best to use hardware to control it rather than software. Really, prob best not to use raid on your pc, unless you spend the real money on a raid controller ($1000+), raid can be a real pain, drives, hardware, software, problems all over.
It's easier to just use the extra drives as mass storage devices and go with something like
StorageProtect for your backups on a PC.