How to fix ‘The mirror device “FilePath\FileName” and the mirror device “FilePath\FileName” have different device specifications’.

Msg 3212, Level 16, State 0, Line 1
The mirror device "PATH\test.bak" and the mirror device "PATH\test.bak" have different device specifications.
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.

If you got this error you probably tried to mirror your backup, but the two disks have different sector sizes.

If you’re running SQL Server 2017, the first thing you need to check is that the instance is patched to at least CU10.

To get the sectorsize of a disk you need to open the Commandprompt or Powershell as Admin, you then run this command:

(replace E: with whatever disk you’re using)

fsutil fsinfo ntfsinfo E:

You then look at the “Bytes per Cluster” value.

Do this for both disks you want to back up your database to and see which is higher. You then take the higher value and use it in the “BLOCKSIZE” parameter.

Just add this to the backup command you are using, replacing 4096 with the highest value you had. Don’t forget the comma when adding the parameter to you command!

BLOCKSIZE=4096
Your backup command should now run as expected!