Cisco are really poor at documenting the hardware aspects of their MCS servers.
You can use the "show hardware" command at the server console to show the status of the drives.
With 7845 servers (HP variant) I have replaced disks on the fly without any trouble.
It generally takes approx 30 minutes to mirror a disk on a new system.
7825 servers do not support hot swap so you have to reboot and go into the drive management utility to set the replacement options.