Drobo 4-bay Gen 2 Firewire. Unmounts while backing up

I’ve bought a 5D3. I have about 2.7TB data on a Gen2, 4 bay Drobo.
I also have plenty of external drive capacity to help me backup and transfer data to the new Drobo.
I will use the old SATA drives from the Gen 2 in the new 5D3, recognising that I cannot swap them in directly.

I am using a late 2012 Mac with USB 3.0, thunderbolt 1.
I have connected my old Gen 2 via a thunderbolt to FW800 adapter.

The problem is…whether I use FW or USB 2.0, the Drobo seems to periodically decide to unmount, throwing whatever backup software I use into disarray.
This usually throws Finder into a frozen state. Unplugging the power from the Gen 2 and plugging back in seems to revive and re-mount the volumes on the Drobo.

When it is working well, I get about 25MB/s transfer speed.

Any suggestions for smoothly backing up? Been trying for days.

Thanks all.