To answer your last question first, YES. Once a RAID (Redundant Array of Independant Drives) is implemented, the only way to upgrade is to replace both drives. I don't know how the RAID systems are in the home PC market (I haven't implemented one in a home system), but in a server you can't just snap in a new drive to add more space. The way a RAID works is that it uses two (or more) drives and writes part of the information sent to the HD to each of the drives (so with two drives, one would have 1/2 of the information the other would have the other 1/2).
I assume that with a home pc it may be possible to use a standard ata connection for additional drives, but truthfully, I don't know for sure. All I know for sure is that once a RAID is implemented you must replace any drives in that array with the exact same drive or the RAID fails and you have a much larger problem. If it is possible to add additional drives, they would work just like a regular HD and the RAID would need to remain unaffected or (in the case of RAID 0) you will lose everything on both drives.
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