12G SAS drives in 6G JBOD issue?

Hi all, ive been running a Promise J630s 16bay 6G SAS JBOD, with Seagate ST4000NM0023 disks (4TB SAS 6G).

All with a 6G LSI SAS controller and TrueNAS.

Ive just purchased 16x Dell branded Seagate ST8000NM0075 8TB SAS 12G disks.

New drives installed and they are not recognised at all. So they dont even spin up inside the JBOD. Ive put them in the JBOD without the caddy to check the connection is ok, and all seems fine. But no power at all on any off them.
6G SAS and SATA drives work fine in the JBOD.

This is my first fooray with 12G SAS disks and though that 12G disks with with anything 6G?

Any pointers? I cant test these drives any other way. But was told all 16 were working ok.