Hard Drive Upgrade on Server 2000

I need to upgrade our HP e800 server, currently we have two 18.8 gig (mirror) hard drives, I purchased two new 37 gigs to replace them. I tried to just remove one old drive, insert one new drive, break the mirror and redo the mirror to the new drive. “C” (system only) “D” (which is data only) mirrored fine but left 20 gig unallocated. I would like to have 10 gigs on “C” and the remaining on “D” without having to reinstall Windows 2000 SBE and take the risk of data loss. What is the best why to go at this?