I actually backed up my OS hard drive data by just taking it putting it into external hard drive case.

My main concern is the data on RAID-ed hard drives. Will I be able transfer them to an external hard drive (assuming it is formatted as FAT) using Knoppix without reinstalling the OS?

I agree with you on RAID 0. My friend built this system for me years ago and I sort of let him do what he wanted. Since then I have realized the pitfalls of RAID 0 and on my second build with I built myself I didn't do any sort of RAID but rather just systematically back up my stuff.

thanks