"Add Mirror" greyed out when replacing a failed Raid1

I am trying to replace a failed disk in a raid 1 under windows 7 pro. I had to physically remove the disk before the machine would boot up properly. I then went to disk management and removed the failed mirror. I then installed the new disk, initialized it as a MBR disk and converted it to Dynamic.  I set up a partition of the same size as the original disk, but when I right click the original disk, "Add Mirror" is greyed out. If I delete the partition "Add Mirror" becomes available but when I try to add the  unallotted space to the mirror, I get an error message that the disks have to have the same partition size and sector size and type.  What am I doing wrong. Thanks for any and all help.