Drobo 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)?



This should help to explain:

If Disk Space Exceeds Volume Size

If you installed a drive in Drobo and you are now being prompted to format, this is because the total storage offered by the inserted drive(s) now exceeds your volume size. The Drobo Dashboard will automatically create a second volume which appears as another volume icon on the Mac’s Desktop or as a new drive letter or partition on the PC. You will be asked to format and name the new volume. Formatting the second volume/partition will not affect the first.

If you are using a DroboPro or DroboElite, you will be prompted by Drobo Dashboard to create another volume through the Volume Management facility.

Thin Provisioning
Current model Drobo’s support 32TB & 64TB Volume and/or Storage size. For more information, see “Maximum Volume and Largest Size Drive

Some legacy Drobo models allowed you to format your Drobo device’s volume size as 1TB, 2TB, 4TB, 8TB or 16TB (even without having that much disk space). This ability to format your volume to be larger than available disk space is called Thin Provisioning. Thin Provisioning allows you to grow into a single large volume over time rather than needing multiple smaller volumes. However, keep in mind some older operating systems have a 2TB per-volume limit.

Note: Even though your operating system may prompt you to format your volume, always use Drobo Dashboard for formatting. Drobo Dashboard allows you to format your volume to a size larger than the available disk space (thin provisioning).

Maintaining a Single Volume

If you have formatted your Drobo legacy product for a smaller volume size and now wish to have one single volume you will need to backup your data onto another Drobo or storage device, reformat the original Drobo device to the volume size of your choice, and then move the data back. Many customers choose the largest capacity available during initial setup to avoid having to move such large amounts of data around.

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