Failing to expand Raid1

Hello,

I have dell power edge T640.

It has raid 1 which is running out of free space. I have purchased two 4 tb ssd which I wanted to replace one at time. I removed one old disk and added new. It shows ready but did not come online by itself. 

Tried from BIOS raid configuration, but did not find an option to bring it online. I tried assigning it as GHS but that did not work. I have iDrac9 express which does not storage configuration menu.

Thanks

Rajeev