Two Volumes on one Drobo, now what?

Usual suspects:

Drobo Gen 3, 2x10TB, 2x6TB
Firmware 3.5.5
macOS Sierra
two volumes

Drobo Dashboard shows:
Drobo Capacity 1

Drobo Volumes 1

So my question is to which volumes do I store content? Both, until they both reaches 20% free (one sooner than the other)?

I’ll be adding 2 more 10T drives to have a total of 4x10T.
Using Drobocal’s 5-bay Drobo as though it were a 4-bay, it shows:

Drobocal 4x10 1

Is this a workable configuration? I don’t really remember the how or why I made two volumes in the first place. I think to change the 16T/16T limit, I’d have to move all the data off those drives and reformat to a higher limit? I can’t upgrade the firmware yet as my current OS, then it won’t see the Drobo.

(Is there a preview button for posts? I have no idea what this will look like.)

2 Drobo Volumes is workable, but the efficiency of the overall storage capacity is not as robust if it was one drive.

You are correct, that to migrate to a single volume, you would have to reformat your current array, which deletes all your current data. Assuming you already have an external copy of your Drobo data, after checking it is up to date and readable, you could start the reformat process, with your 2 new 10GB HDDs in place. Alternatively you can copy your existing data onto your new 10GB drives and use that to repopulate your array after the reformat, then ‘upgrade’ your 6GBs to 10GBs.

EDIT: When you reformat, make sure you choose the 64GB Volume size, allowing you to sequentially upsize your HDDs at a later time without disrupting your data

A recommendation is to always keep an external copy of your data up to date. The single point of failure in a Drobo (and any RAID machine) is the unit itself.