One drive fails to join VDEV

[stajo]

I am setting up a 10 disk VDEV with older 1,5 Tb disks, 7 of them is WD Greens flashed to fixed revs, two Seagete Barracudas and one Samsung 5400 rev. Not ideal but its a first test. One of the WD Greens creates a warning: "Mixing disks of different sizes in a VDEV is not allowed" as soon as I put that one in any configuration with another disk. If I look at the disk information under Disks it shows the same data as all the others, 1,36 TiB and it is even the same batch as some of the others, WDC WD15EARS-00MVWB0. It has passed through long and short smart tests and has been wiped short and long with zeros. What can be wrong?