This guide will talk about rebuilding a failed RAID 1 disk with WD My Book Duo on macOS, it should also works on Thunderbolt Duo or other RAID 1 setup.
Many other guides only tell you how to replace the whole two disks without restoring / rebuilding data for this common situation:
In a common case, your RAID 1 setup could fail with only one defective disk, while the other is online. If you see this status on your macOS:
- Buy a brand new disk, same size and model as your failed disk.
- Reboot your mac in recovery mode, this step is very important to make sure your disks are not used by macOS.
- In recovery boot, open Disk Utility.
- Rename your RAID set, for example if your RAID set is
External-Raid, rename it to something likeExternal-Raid-Rebuildor just some names different than your original, this is the most important step to make sure your RAID set wouldn’t be using after entering macOS system. - Restart your system in normal mode.
- Login to your system, then open Disk Utility, congrats, you will see the new disk is now rebuilding.
Other notes:
- Restraint editing/adding/removing files on your RAID set while rebuilding.
- The rebuild process could take hours depends on your disk size (10-20 hours for a 4 TB + 4 TB RAID 1 setup).
- After the rebuild complete, you can rename your RAID volume name back to original.
This is not the only way to rebuild your RAID with a failed disk. According to WD documentation, you can power on your My Book Duo without connecting to macOS (remove the thunderbolt cable), then the My Book Duo should rebuild it automatically. However, it’s really hard to know when the rebuild process will finish, there’s no special indicator status for this situation, so I prefer rebuilding in macOS. This could be the best method for me.