Daisy chain

I have a two drive Thunderbolt 3 RAID operating as independent drives. I also have a G-Tech 6TB Thunderbolt 3 that I want to daisy chain with the RAID to back up one of the Raid Drives.
The new iMac is not recognizing the external drive. How so I get the imac to recognize the drive?

You need to make sure you are using the correct cables and plugging into the correct ports.

On the Mac it needs to be the TB3 port which has a lightning bolt on it, and on the drive side its the same. Make sure you plug them all into the lightning bolt ports that mean Thunderbolt.

Done that.