EX 2 Ultra -> adding disc problem

I’m using EX2 Ultra with 2T disc in Disc 1 slot.

Later I bought WD Red 4TB 3,5" SATA3 256MB to install as Disc 2 slot.

Intention is to operate NAS in RAID 1 configuration.

Problem:LED diod for Disc 2 slot turns red all trhe time.
Disc 1 in slot 1 works normally.
Where is the problem?