Error: "All disks holding extents for a given volume must have the same sector size, and the sector size must be valid."

I had this error too, and managed to get passed it by doing the process from DISKPART instead of disk management GUI.

To do that, open DISKPART from a command prompt, then (assuming disk 1 is the source drive, and disk 2 is the destination drive)

rescan
select disk 1 
convert dynamic
select disk 2
convert dynamic

list volume
(then find the volume number for the partition you wish to mirror)
assuming this volume is 1:

select volume 1
add disk=2

That should do what disk management coudn´t

The problem is that I was starting with a drive full of data, and wanted to add a mirror of said drive.

This will not always work, and the error it gives is meaningless at best, and misleading at worst.

A solution is to start with two unallocated volumes, and select "New Mirrored Volume":

There are loads of videos on YouTube which describe how to do this, the one that I successfully used was Set up a Mirrored Array in Windows 7.

Update

It is possible to avoid this error, see the answer from @CyberPonk.