Fix ERROR_NOT_QUORUM_CLASS (0x000013A1) in Windows Failover Cluster

You're setting up a Windows Failover Cluster on Server 2019 or 2022. You add a shared disk to the cluster's available storage, then try to assign it as the quorum witness or add it to a role. Instead of success, you get ERROR_NOT_QUORUM_CLASS (0x000013A1). The disk seems fine — it's online, it's visible on all nodes — but the cluster refuses to use it.

What's actually happening here is that the cluster's storage subsystem checks the disk's underlying characteristics before accepting it. Windows Failover Cluster requires disks to be GPT (GUID Partition Table) formatted, not MBR (Master Boot Record). If the disk is MBR, or if it's a basic disk without a valid partition table that meets cluster storage class requirements, the cluster rejects it with this error. The reason step 3 works is that converting the disk to GPT satisfies the cluster's storage class filter, which is hard-coded to only accept GPT disks for quorum and cluster shared volumes.

Root Cause

The cluster storage class filter has three requirements:

• The disk must be GPT — MBR disks are blocked.
• The disk must have at least one partition (simple or spanned).
• The disk must be online and writable on all nodes.

If any of these fail, you get 0x000013A1. It's not a permissions issue, not a driver issue — it's a partition style mismatch.

The Fix

1. Check current partition style — On any cluster node, open Disk Management (diskmgmt.msc). Right-click the disk in the bottom pane, select Properties, then the Volumes tab. Look for "Partition style." If it says "Master Boot Record (MBR)", that's your problem.
2. Back up any data — Converting from MBR to GPT destroys all data on the disk. If there's anything on it, copy it elsewhere first.
3. Convert the disk — Open an elevated Command Prompt or PowerShell. Run:

   diskpart
   list disk
   select disk X
   clean
   convert gpt
   create partition primary
   format fs=ntfs quick
   assign letter=Q
   exit

   Replace X with the disk number. The clean command wipes it. convert gpt changes the partition style. Then create a single primary partition and format it.
4. Bring the disk online on all nodes — In Failover Cluster Manager, go to Storage > Disks. Right-click the disk and select "Online." If it's already online but stuck, right-click again and choose "Bring Online."
5. Add the disk to Available Storage — Right-click the disk and select "Add to Cluster Shared Volumes" or assign it to a role. The error should be gone.

Still Failing? Check These

If the error persists after conversion, you've got a different problem:

• Disk signature conflict — Two disks with the same signature? They'll both fail. Run diskpart, uniqueid disk to verify each disk has a unique identifier.
• Disk is read-only — Check the disk's attributes. In Disk Management, right-click the disk, Properties, check if "Read-only" is checked. If yes, disable it.
• Cluster validation — Run the Validate a Configuration wizard in Failover Cluster Manager. Select all nodes and the disk. Look for warnings about storage. If it says "The disk's partition style is not supported", you missed the GPT conversion.
• Wrong disk type — Cluster requires a dedicated shared storage device (SAN, iSCSI, or SAS JBOD). USB drives, internal SATA disks, and Windows Storage Spaces disks won't work for quorum — they'll throw 0x000013A1 or similar errors.

One more thing: if you're using Windows Server 2012 R2 or older, you might hit a similar error with different numeric codes (like 0x000013A0). The fix is the same — GPT conversion. But always check your specific OS version because the cluster storage stack changed in Server 2016 and later.