Different sized disks in the same Vdev


I’m new to TrueNas CORE and am trying to set it up. I’ve got an old PC with 3 drives. 1 SSD (boot drive) and 2 HDD’s (1 of 4tb and 1 of 8 tb).
I’m trying to set it up in a way that the 4tb drive will be the carrier of the main data pool, and the 8tb will be a mirror of the 4tb one (I know it’s inefficient).
When i try to set up my pool I get an error message because the drives have different sizes. How can I overcome this and use this setup like I want to?
Is it an option to parition the 8 tb drive or something?

The error message should come with a ‘Force’ button of some kind.

Alternatively, create the vdev with the 4 TB drive as the sole member, and then extend the vdev with the 8 TB drive to make it a 2-way mirror (Storage>Pool>gear>Status>drive>…>Extend).


Did you add the 4TB drive or the 8TB drive first?

As @etorix said, there should have been a “Force” checkbox to allow you to create the pool anyway. A screen shot of the exact error would be helpful, particularly if you don’t see that checkbox.

No. At least, not through the GUI–you could do it at the shell, but that would be strongly discouraged.

The force button did the trick although there was a big warning stating that “mixing disks of different sizes in a vdev is not allowed”. I’m guessing/hoping it works now as I forced it to make 1 vdev with 2 disks of different drives on the mirror mode? Is that correct?
Mixing Disks

“Not allowed” is a bit too strong a warning: It works, as two 4 TB drives.