The command you selected is not available

Hi guys... here's a good one.
I am running Windows XP Pro Version 2002 w SP2 on my development workstation
at the office.
Last week I had an unmirrored drive on a file server go south. It created a
bunch of problems. Even though it was the "testing" server, I had to replace
the drive and recreate the testing environment. I decided that henceforth
there will be no unmirrored drives on any servers here even if they are
"non-critical". I proceeeded to make that happen last week.

This week I decided to add an identical Sata disk drive to my Development
workstation and create a Windows RAID 1 mirror. The existing drive has the
Windows Sysytem on it and all my development software, etc.

In Disk Management:

I see Dynamic Disk 0 (the System drive) 34.46 GB NTFS Healthy with 8 MB
unallocated.
I see Dynamic Disk 1 (the new drive) 34.47 GB all unallocated.

The disks are identical Western Digital models.

When I right-click either Dynamic Disk the Add Mirror selection is grayed
out.
So I tried using DiskPart in the Command Prompt.
I followed all the steps as outlined.
When I get to the part: DISKPART> add disk 1
I get the following response.

"The command you selected is not available with this version of Windows."

Anybody know whats going on?
TIA......