Drobo unexpectedly created 2 volumes / "The Disc you attached was not readable" (macOS Big Sur)

I have a Drobo Gen 2. I have recently replaced one 2 TB drive by a 4TB drive (other drives are 4TB, 4TB and 2TB). The update process seemed to take place correctly (after hours, all 4 drives became green). However, since then if the Drobo is connected to the computer when I boot it, I get an error message from macOS: “This Disc you attached was not readable”. I also see “Trusted Mass Storage Media” twice in disk utility. Both instances are for 8,8 TB. The only difference between the 2 is the BSD device node.

In Drobo Dashboard I also see 2 volumes (where I had only one before I think), and I got the message that one was not formatted. Normally I should be able to have a 16 TB volume instead of 2. So:

  • are there any ideas on how to get rid of the error message?
  • is there a way to merge the 2 Drobo volumes without losing the content on the first volume?
  • As a last resort, should I just format the 2nd volume (in Drobo Dashboard and not in disk utility I assume)?