G-RAID w/TB2 disappears on iMac restart

I have an 8Tb G-RAID with Thunderbolt 2 running as 2 separate drives (2x 4Tb JBOD) daisy chained through a Drobo 5D into a 2011 27" iMac running Mavericks.

It works fine until the iMac is restarted, then neither drive will appear until I power down the G-RAID box and turn it back on again.

Any thoughts?

Likely due to the fact that the iMac has a first generation TB1 connection on it and it isn’t activating the newer TB2 interface upon boot up.