Manage disks grayed out

[Pepet]

Hi there!

I' need to add a new HDD to an existing volume (which has 2 disks already), and I can't find a way. As you can see, "manage" tab is grayed uot:

 

 

I've seen many times that this is usually because the new disk has to be equal, or bigger, than the biggest in the volume. Again, you'll see that this is not what happens here, I'm trying to add a bigger disk:

 

 

I'm using Xpenoboot, hardware installation, if this could mean anything.

 

Could I get a clue from you, guys?

Thanks in advance!