How to recover and mount a missing partition on an external 12 TB hard drive with Mac Studio?

If it is an APFS volume, then run First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container(s) appear on the left pane of Disk Utility. Running First Aid on the hidden Container (assuming an APFS file system) will also run a scan on all other APFS volumes located within that Container.

Even if the First Aid summary says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors exist. If there are errors, then run First Aid again until the errors are gone. If after several scans the errors remain, then Disk Utility is unable to repair the file system. In that case, knowing the file system used may allow for other options.

FYI, you should have frequent & regular backups of any important data stored on external media (including the cloud).....in addition to the data on the internal SSD.

You can try checking the health of the external Hard Drive by using DriveDx (free trial period) and posting the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. You will need to install a special USB driver in order to access the health information on the external drive.

A few missing bits of information can be helpful:

• Based on what you provided, is this a Western Digital (WD) or a SanDisk RAID drive?
• Is it currently setup in any RAID configuration prior to you initially partitioning it?
• What partitioning scheme are you using for each partition?
• What format (APFS, NTFS, BFRS, etc.) is the partition that you can't mount on your Mac?
• Just to be sure, do you have two physical partitions or a single partition with two volumes on the drive?
• Have you tried to access this drive from another computer?
• Any other information you can provide us?
• When you spoke with Apple Support, did you happen to escalate the issue beyond first-level support?

Robert Rosenberg3 wrote:

I did try those things. First aid was unable to read the partition so it closed almost immediately. What seems to be the problem is the partition isn't openable and needs repair. I am unable to find a way to repair the partitioning of the drive.

In that case, your option left is to copy everything you can from all partitions and volumes on the drive and erase/reformat the entire physical device. Then set it up again the way you want it and restore those files. Presumably you have a backup for everything in the lost partition. I am confused a bit because I thought RAID was to protect you from such failures. But backing everything up to separate physical devices is really the only safe thing (as HWTech points out).

I would ask how old is this 12 TB drive, is it mechanical or SSD, have you tried to diagnose its hardware with DriveDx or other similar tools? A sudden complete loss of a partition ... it seems like some kind of physical mishap within the drive has occurred, I am not sure I would continue using such a drive. Beware using very large storage devices and partitioning them in smaller pieces. That exposes you to higher risk of losing many things at once, versus one thing, because if the physical device is suspect, so are each of the smaller partitions or volumes or containers that are on that physical device.

Robert Rosenberg3 wrote:

I did try those things. First aid was unable to read the partition so it closed almost immediately. What seems to be the problem is the partition isn't openable and needs repair. I am unable to find a way to repair the partitioning of the drive.

If they are actual partitions as opposed to APFS volumes, then try running First Aid on the physical drive.

I highly recommend checking the health of the drive using DriveDx as I suggested previously.