I'm trying to mirror drives of the same model, but one appears to be slightly larger

I'm trying to mirror a drive using Disk Management in Windows Home Server 2011. The target drive (Disk 1) is the exact same model as the source drive (Disk 0), but the target drive appears to be 1MB smaller in size.

Disk 1 is unallocated. From what I can see it has the same MBR partition style as Disk 0.

What could be causing the discrepency? And how can I solve this without shrinking Disk 0?

Thank you

Add Mirror... is greyed out

Source disk was left with 1,846,187MB after allocating 100MB for a System partition and 60GB for Windows.

After mirroring the System and Windows partitions to the target disk, it is only left with 1,846,186MB.

Solution 1:

I was able to mirror the drives in DiskPart. At this point, all I had to do was select the volume I wanted to mirror, mirror it and let it resynch. And that's what it's doing right now.
select volume YOUR_VOLUME_#
add disk=TARGET_DISK_#

If you want to do it from scratch, assuming Disk 1 is the target disk and Disc 0 is all your stuff, something like this would do it:
select disk 0
convert dynamic
select disk 1
clean
convert mbr
convert dynamic

list volume
select volume THE_FIRST_PARTITION_#_OF_DRIVE_0
add disk=1
select volume THE_SECOND_PARTITION_#_OF_DRIVE_0
add disk=1
select volume THE_nth_PARTITION_#_OF_DRIVE_0
add disk=1

That should work. I don't know why the GUI sees the drives differently.