I'm running Windows 2008 Enterprise on a single disk desktop with a 500gb Hitachi HDD. I tried to add a Seagate 500gb HDD for mirrroring, but when I tried to add the unallocated Seagate, I got the error message: "all disks holding extents for a given volume must have the same sector size". I ran the FSUTIL command and the information shown for both drives is identical with the exception of the MFT start zone address. ( both have 512 sector size). I noticed the capacities were about 1mb different (with the new drive being larger). Could this be the issue? and if so, what I can do about it? BTW, I seem to remember mirroring resizing the added unallocatted drive, so why doesn't it do it now?