For the last few days I was having an issue with my Storage Spaces setup. I have a Two-way Mirror Storage Space comprising of 2 identical SATA-attached HDD drives.

However the Storage Spaces in the Control Panel is showing a Warning without giving any explanation of what is actually wrong (no tooltip or anything). In addition, it shows different values for the used capacity! How can that be in a mirrored, identically sized disks?

Storage Spaces CP

Running a few diagnostic commands in PowerShell does not provide much info either:



They basically duplicate the what the Control Panel was displaying. However what struck me was that it showed storage efficiency at 73.98%! This should be exactly 50% for my setup. Does it imply then that a substantial part of the data is out of sync then? If so, how can I resync it, making sure it "knows" which data should be used?

  • I have write caching disabled for both drives, so in all cases writes should be immediately reflected on the disks.
  • I did use a Hex Editor with raw disk viewing support to check if indeed it mirrors the data. I tried to find very recent files that I have created in the NTFS MFT using Hex Editor, and was able to do so, with the file records for the new files existing at identical sectors on both drives! I also checked at random locations to see if the data is identical between the drives (sector-for-sector), and it appears to be so. However, I am still unsure if it applies to all data. After all why Control Panel and PowerShell report strange values for % of pool used and storage efficiency?
  • I am using Windows 10 1803 x64 Professional

Here's other relevant info:

PassThroughClass                  : 
PassThroughIds                    : 
PassThroughNamespace              : 
PassThroughServer                 : 
Access                            : Read/Write
AllocatedSize                     : 4049885724672
AllocationUnitSize                : 268435456
ColumnIsolation                   : PhysicalDisk
DetachedReason                    : None
FaultDomainAwareness              : PhysicalDisk
FootprintOnPool                   : 8100576755712
FriendlyName                      : SS1
HealthStatus                      : Warning
Interleave                        : 262144
IsDeduplicationEnabled            : False
IsEnclosureAware                  : False
IsManualAttach                    : False
IsSnapshot                        : False
IsTiered                          : False
LogicalSectorSize                 : 4096
MediaType                         : Unspecified
Name                              : 
NameFormat                        : 
NumberOfAvailableCopies           : 
NumberOfColumns                   : 1
NumberOfDataCopies                : 2
NumberOfGroups                    : 1
OperationalStatus                 : OK
OtherOperationalStatusDescription : 
OtherUsageDescription             : 
ParityLayout                      : 
PhysicalDiskRedundancy            : 1
PhysicalSectorSize                : 4096
ProvisioningType                  : Thin
ReadCacheSize                     : 0
RequestNoSinglePointOfFailure     : False
ResiliencySettingName             : Mirror
Size                              : 5992553119744
UniqueIdFormat                    : Vendor Specific
UniqueIdFormatDescription         : 
Usage                             : Data
WriteCacheSize                    : 0

Any help would be appreciated. Thanks.

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