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Overview: WD My Book Thunderbolt with 2x WD 4TB drives (factory-fitted), connected to 2014 iMac (macOS Sierra 10.12.6) via Thunderbolt 2 port. Drives are configured as RAID 1. Both drives are listed as Online in Disk Utility. The drive set (i.e. combined volume) name is listed as -- in Disk Utility. I am not able to mount the drive set using the Mount button. The Mount button is not greyed out. I am not provided with any UI feedback after selecting the Mount button.

Disk Utility screenshot

Using Terminal, I can see the drives are Online and I can also see the drive set name (My Book RAID0 in the excerpt below -- note: drive is using RAID 1, not RAID 0; the name is not correct, but it is expected).

I have an additional My Book Thunderbolt connected that is functioning correctly. Terminal commands show both My Book units connected, each with 2x physical drives in Online state, and the respective set in Online state.

$ sudo diskutil appleRAID list
Password:
AppleRAID sets (2 found)
===============================================================================
Name:                 Overbusiness
Unique ID:            81414396-C39C-47B5-B893-6DAC40A80561
Type:                 Stripe
Status:               Online
Size:                 4.0 TB (4000109887488 Bytes)
Rebuild:              manual
Device Node:          disk7
-------------------------------------------------------------------------------
#  DevNode   UUID                                  Status     Size
-------------------------------------------------------------------------------
0  disk4s2   261173D8-FE72-4FE2-B6BE-B35B0B3E29D8  Online     2000054943744
1  disk2s2   8D7ECA83-8E1F-4FAF-BAC1-3E275A41D8CB  Online     2000054943744
===============================================================================
===============================================================================
Name:                 My Book RAID0
Unique ID:            515F3E4B-AD3C-4043-8351-33F715E56F05
Type:                 Mirror
Status:               Online
Size:                 4.0 TB (4000443039744 Bytes)
Rebuild:              automatic
Device Node:          disk6
-------------------------------------------------------------------------------
#  DevNode   UUID                                  Status     Size
-------------------------------------------------------------------------------
0  disk1s2   E117DB15-AE80-49CB-84DA-B468569DB4EF  Online     4000443039744
1  disk5s2   47701813-0AB1-4317-B769-358B30E14702  Online     4000443039744
===============================================================================

I have tried to mount the drive using its Device Node in both standard and read only modes (sudo diskutil mount /dev/disk6 and sudo diskutil mount readOnly /dev/disk6). I have tried to mount the drive using its Unique ID (sudo diskutil mount readOnly 515F3E4B-AD3C-4043-8351-33F715E56F05). Unfortunately, neither of these options worked. I did receive some error text in the Terminal which I omitted to note, but I will re-run the command later and update this post with the error text.

Question: given the above scenario, what can I do to manually mount the RAID?

Disclosure: I've purchased commercial data recovery software to recover the drive contents as time is tight, but I'd like to understand what actions I can take to manually resolve this in the future should it happen again. I will have some time after the data has been retrieved (estimate: 12-18 hours after this post timestamp) to work on the RAID in isolation, and I will provide feedback where I can.

Thank you.

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