It seems that the problem was in my choice of USB port when I installed my new Drobo 5D.
The only spare USB3 port was on the front of my PC. However, it turns out that this is not a native motherboard USB port, but on an internal hub. The Drobo doesn't work well with some hubs, including mine it seems.
So, I swapped over my Drobo 5D (front port) with my USB3 hub (rear port) and everything is happy, it seems. My backup (copying old Drobo V2 to new Drobo 5D) has been running 24 hours, with no errors.
The only spare USB3 port was on the front of my PC. However, it turns out that this is not a native motherboard USB port, but on an internal hub. The Drobo doesn't work well with some hubs, including mine it seems.
So, I swapped over my Drobo 5D (front port) with my USB3 hub (rear port) and everything is happy, it seems. My backup (copying old Drobo V2 to new Drobo 5D) has been running 24 hours, with no errors.