Copying Time Machine backup to second Drobo repeatedly fails

I have two Drobo 5Cs, each with five 4TB matching drives in them. They are hooked up to a MacBook pro via USB 3.

I am in the process of migrating a Time Machine backup volume from one drive to the other Drobo.

It is about 2TB in size, and will copy for DAYS and then fail.

The first two times, after about 2+ days, one of the Drobo just ejected itself, ending the backup.

The third time it has a different error that I wasn’t able to capture, but the volumes were both mounted.

I have now spent a week of time waiting to copy. Are there known issues trying to move large copies between Drobo?

Time Machine has a notoriously data-flooded cross-linked folder structure.
I would recommend copying it off onto an external HDD, then transferring the same back to your other Drobo from the external HDD.