Hmmmm...well...I've found reference to plenty of folks who have increased RAID5 capacity by adding drives, though they're all using far more expensive and advanced RAID management systems than I'm likely to have. And...looks like RAID5 controllers for handling 5+ disks (so I can add drives, if possible) are mucho expensive. So...looks like I'll go with RAID1 for now and add a second array when I require additional space.