0xC038005B Mirror volume fix – disk partition style issue

Yeah, that error is annoying. You're sitting there, disks are healthy, dynamic, ready to go — and Windows just says "nah." I've seen this hit after a fresh Server 2016 or 2019 install when someone tries to set up a mirror on a second disk that's still MBR.

The quick fix: Convert the target disk to GPT

Open Disk Management (diskmgmt.msc).
Right-click the disk you want to mirror onto — the one that's not the boot drive — and choose Convert to GPT Disk.
If that option is grayed out, you've got data on it. Back it up first, then delete all volumes, and try again.
After conversion, both disks must be dynamic. Right-click each disk and pick Convert to Dynamic Disk if they're not already.
Now right-click the volume you want to mirror, choose Add Mirror, and pick the GPT-converted disk.

That's it. The mirror will start resyncing in seconds. I've done this on maybe two dozen servers — it's never failed when both disks are GPT dynamic.

Why this works

Windows Volume Manager (VolMgr) has a hard rule: mirrored volumes must sit on disks with the same partition style. If your boot disk is UEFI (almost always GPT on Server 2016/2019), and the second disk is still MBR, VolMgr throws 0xC038005B. It doesn't tell you that directly — it just says "not supported." The real problem is a partition table mismatch.

MBR is limited to 2TB and max 4 primary partitions. GPT handles modern drives and is required for UEFI boot. Microsoft quietly tightened this in later builds of Server 2016 — you can't mix styles in a mirror anymore. No workaround exists for keeping MBR. Convert it and move on.

Less common variations

Both disks already GPT but still get the error

One of them might be read-only or have a bad sector. Run chkdsk /f on each disk. If that doesn't fix it, check the disk's status in Disk Management — if it says "Failed" or "Not Initialized," replace the disk.
Sometimes a stale driver causes this. Update your storage controller driver (especially on HP ProLiant or Dell PowerEdge with RAID controllers).

Error appears in Server Manager, not Disk Management

Launch diskpart from an admin prompt. Run list disk — verify that both disks show Dyn and Gpt under the Flags column. If one shows MBR, use convert gpt after cleaning it (data loss — backup first).

Mirror works initially, then fails after reboot

This is usually a missing or corrupt Volume Manager driver. Open Device Manager, expand Storage controllers, right-click Microsoft Storage Spaces Controller (if present) or Standard NVM Express Controller, and update the driver to the latest from your OEM.
Alternate cause: the second disk is connected via a HBA or external enclosure that doesn't support dynamic disks. Move it to an internal SATA or SAS port.

Prevention

Before you even start:

Check your boot method. If it's UEFI (which it should be on anything from 2016 onward), convert every mirror-ready disk to GPT before making them dynamic.
Use diskmgmt.msc or diskpart to verify partition style first. Don't assume a new drive out of the box is GPT — most are still MBR from the factory.
Keep your storage drivers up to date. I've seen buggy OEM drivers cause VolMgr to fail even on perfectly good disks. A driver update fixed it every time.
If you're building a new server, set all data disks to GPT during the initial OS install. Saves you from this headache later.

One last tip: if you're still on Server 2012 R2, this error is rarer because it supports mixed partition styles in some scenarios. But don't rely on that — upgrade to 2016 or 2019 and use GPT consistently. You'll thank me when a disk fails and the mirror rebuilds without drama.