the disc you attached was not readable by this computer

We can better help you if we knew:

Your iMac's sub-model and OS version (from "About this Mac...")

What disk? A CD or DVD? An external hard drive?" A USB "thumb" drive?

If an external hard drive, what make and model?

We can guess, but we are not great guessers. Things usually go to Perdition fast when we start making assumptions!

I think the issue is relating to the "TRUSTED Mass Storage Media".

It is the only drive with no partitions, and therefore, not readable. Try adding a usable partition and see what happens.

Cheers,

Jack

Yes. That drive is the issue.

In this case, the only way to add partitions is to try to use the "Erase" button on the disk. If you pick this route, use a name, pick APFS, and GUID.

Cheers,

Jack

Hi Jack, the problem seems more a driver issue: physically and logically there is only one volume (I don't have 2 disks "Trusted Mass Storage Media"). It is just that there seems to be a wrong allocation of the BSD device node (see screenshot below: the only difference in the 2 TRUSTED Mass Storage Media entries in Disk Utility is the BSD device node.

In addition, disk utility shows the same thing whether or not I connect the Drobo before or after booting while the error message of "disk attached was not readable" only appears if it is connected before booting.

I think I will contact Drobo.

Thanks a lot.

Cedric

PS: UPDATE: in Drobo Dashboard there are 2 volumes shown, one unformatted. So yes I need to format or merge the volumes... (I am waiting for a reply from Drobo on how to do that)

I have the same issue with an iMac 5K (late 2014), under macOS Big Sur 11.2.1. When I click on Initialise to know which disk the message relates to, it shows that the issue comes from an APFS System Snapshot (com.apple.os.update-CExxxxxx).

Is there a way to delete the snapshot safely to get rid of the message?

I am trying to further pin down the issue as it may be linked to external hard drives too: I rebooted without a drive connected and did not get the message. I will try to reboot with my different external drives (one at a time) to see if there is one in particular that provokes the error.

Thanks Jack. Indeed there seems to be an issue with my Drobo which provides the 2 "TRUSTED Mass Storage Media". What is weird is that if I connect it after booting, then I get no messages, but if I boot with the Drobo connected then I get the error message.

In both cases, the view in Disk Utility is the same.

And unfortunately there is no way I can add a partition in the TRUSTED Mass Storage Media that now appears without a partition (icon greyed out probably because this is just a duplicate of the previous one).

I am starting to think there is an issue with the driver of the Drobo.

The snapshot is needed by the system, as it is your boot drive. It cannot be deleted. Can you post a screenshot of the message?

Cheers,

Jack

Here is the error message.

And now the disk utility window.