You would need to expand the volume capacity. It is not expanded by default.
I didn't see the chipset mentioned. You can check if capacity expansion is supported on your chipset. Open the console app (Intel(R) Matrix Storage Console via the Start menu), click View -> Advanced if you're not already in Advanced mode, right-click on your RAID 1 volume in the device tree, and click Modify RAID Volume. I don't have a computer up to check so may have some of the specifics wrong .. but you don't want to change anything, just click through the modify volume wizard until you get to a screen that has some options for capacity expansion (I think it's on the volume size screen). If the option is not greyed out, you can expand the volume capacity that way.
If it is greyed out, you can still do what you want, it's just a little more work. I thought there was web content on how to do this, but cannot find it. The short answer is that you need to create the RAID 1 volume again. You would reset the disks to non-RAID (will just delete the information about the RAID volume, not the data), take one of the hard drives and expand the partition so that it fills the entire disk, then create a RAID 1 volume again using the migration feature (Actions -> Create RAID Volume from Existing Hard Drive). Actually, you probably don't really have to expand the partition, but it's not a bad idea.
If this is confusing, let me know.
Thanks for the thorough reply. My chipset doesn't support volume expansion (ICHR8/9) and I have gotten mixed answers on whether data is lost if I break the existing RAID 1 arrangement. When I get to the point of doing it, Matrix tells me that all data will be lost. So, I just made another RAID 1 array with the unused capacity. It's a little awkward and I used up a drive letter but it works anyway. SinceI've already captured the additional capacity, I think what I will do is the next time one of the RAID drives fail I will use that opportunity to revert to non-RAID, expand, rebuild the RAID 1 with the expanded capacity. I have a USB external drive that is big enough to do a complete backup and Vista Ultimate is supposed to make recovery easy (ier).
Thanks again for the help!