Hello, Generally, when a member drive is disconnected or has failed, it might occur this error. I would like to suggest you troubleshoot as below: 1. Backup the whole Storage Space to a new hard drive in order to avoid data lost. 2. Attach the disks to another computer to identify which disk that had a problem. 3. Format the disk that did not show up in Windows Explorer and was not readable. 4. The reformatted disk was then added to the original Storage Space and the rebuild process started automatically. Since the issue is complex and it's related to important data, it's recommended to open a case with MS, so that they have more channel to access your environment to do detailed check and provide more efficient support. Below is the link to open a case with MS: https://support.microsoft.com/en-us/gp/customer-service-phone-numbers Best Regards, Hania Lian

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Storage Spaces pool will be inaccessible as long as storage jobs are running. Since these jobs can not be accomplished due to the death of the second mirror, you need to stop them forcefully in order to regain access to your data. Please run the following Powershell command using elevated privileges and see how it goes.

Get-StorageJob | Stop-StorageJob

You will probably need to run chkdsk against your remaining mirror to fix possible file system issues that may have occurred due to sudden power loss.