How (exactly) did you add the drive to the Raid 5?  Does the unallocated space show up at the end of the existing disk (disk0 or disk1) or as a New Disk?

Hmmm this should work fine.  You can ONLY extend the last partition.  Is the disk Basic - this won't work for a Dynamic disk.  How about Partition Magic?

Sorry if I left out some details. I popped the new drive into the 1x6 backplane, saw the drive detected, added new member to the virtual disk, the array was reconstructed and resynched, and when that was done, I restarted and saw the space in Disk Management.

In Disk Management, the new hard drive shows up at the end of Disk 0, in both cases. It's right after the partition that I want to add the space to.

If I right-click the disk, it says Convert to Dynamic Disk, so I assume its Basic.

Microsoft has an article on how to use diskpart here.

2 important lines in there are:

" Only the extension of data volumes is supported. System or boot volumes may be blocked from being extended..."

and

" You cannot extend the partition if the system page file is located on the partition. Move the page file to a partition that you do not wish to extend."

Yeah, I thought I made it clear that these are non-system partitions (D: and X: partitions)...the pagefile is on the C: partition on both of these servers.