Mirrored RAID Failed

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Level 1

9 points

Mirrored RAID Failed

I recently set up a Mirrored RAID via Disk Utility. It consists of two 8tb external drives and contains about 5.5 tb of data.

It was working fine until (perhaps coincidentally) the OS upgraded from 10.15.6 to 10.15.7, after which the RAID drive would not mount.

It did not appear in Finder, but in Disk Utility, the RAID drive appeared, but not mounted, and the two member drives appeared grayed out, also not mounted. The mount button returned an 'Invalid Drive' when pressed.

Also appearing, was a new drive within one of the member containers.

None of the repair or test processes worked. Finally, I restored to a spare 8tb external drive from Time Machine. (an 18-hour process)

First of all, any idea of what happened, and was there a way to fix this that I don't know about?

And, I was under the impression that a drive failure on this mirrored RAID would still allow me to access the data on the other drive? That did not appear to be possible.

iMac 27″, macOS 10.15

Posted on Oct 5, 2020 8:39 AM

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Answer 1

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jgwalter Author

Level 1

9 points

Oct 6, 2020 7:04 PM in response to HWTech

Thank you very much for this answer. It confirms what I suspected from the start. This attempt was to be a solution to my third failed Drobo RAID 6. I still have a 4th (NAS) operating, but I've given up on that company.

I have been investigating the Synology solution, as my son uses one for his business. It certainly seems like a more robust product than the Drobo, but I really don't know much about them yet.

Part of the problem that I had with the Drobo were numerous drive failures. Drives that failed could still be used in stand-alone operation, but would not operate in the Drobo. Most of the drives that I used were WD Reds, 5400rpm non-enterprise models, which I think may have contributed to the failures.

However, the worst failures were with the Drobo hardware itself, the latest being only three years old.

Again, thank you for this excellent answer.

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Answer 2

HWTech

Level 9

60,302 points

Oct 5, 2020 8:07 PM in response to jgwalter

I would never use software RAID on two external drives on a Mac. All it takes to break that RAID is for one of the drives not to be available even for a moment or for a drive to take too long to respond. I've even seen RAID mirrors on Mac internal drives break for no reason. If you are going to use a software RAID, then make sure to use it on a drives contained within a single drive enclosure which contains multiple drives in JBOD. At least then all the drives in the RAID are using the same shared controller minimizing the chances of the RAID breaking.

Also you have to be extremely careful on the drives used in a RAID now because the drive manufacturers just started selling drives which use SMR recording methods (Shingled Magnetic Recording) which are not compatible with RAID setups.

https://arstechnica.com/series/smr-where-you-least-expect-it/

Edit: It might be safer to use an external device such as a Synology NAS (there are simple low end options as well as higher end options).

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Answer 3

HWTech

Level 9

60,302 points

Oct 6, 2020 7:56 PM in response to jgwalter

Yes, the Drobo hardware are pieces of junk! As is their proprietary software to manage the Drobo units. I also don't like the Promise Pegasus units either. There should never be a need for proprietary software to manage external storage.

I've never personally used the Synology hardware, but I've read good things and a co-worker has one which has been working well for him for many years now. It would be my first choice if I didn't make my own solutions using Linux.

You should check & verify the health of the hard drives you pulled from the Drobo as the drive may have started getting some bad blocks. You can use DriveDx which will check the SMART attributes on the drive which is the health information gathered by the drive's own hardware. If you are familiar with the command line and don't want to pay for DriveDx, then you can use the free open source utility at the core of DriveDx which is from the smartmontools project. It also means you need to learn a bit more about how to read the SMART attributes and configure the smartctl utility to monitor the drive for you. DriveDx does a good job of assessing the health of a hard drive (not so good with SSDs though).

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