Windows Mirroring error

I’ve been attempting to use the Windows 2019 built-in dynamic disk mirroring but encounter an error on Windows (in Disk Management) “All disks holding extents for a given volume must have the same sector size, and the sector size must be valid”.

Ok, fair enough… However, as far as I can tell, the sector size on the local disk (a small partition on the c: boot drive) is 512:

U:>fsutil fsinfo sectorinfo y:
LogicalBytesPerSector : 512
PhysicalBytesPerSectorForAtomicity : 512
PhysicalBytesPerSectorForPerformance : 512
FileSystemEffectivePhysicalBytesPerSectorForAtomicity : 512
Device Alignment : Aligned (0x000)
Partition alignment on device : Aligned (0x000)
Performs Normal Seeks
Trim Not Supported
Not DAX capable
Error: Incorrect function.

And the TruNAS SCALE disk that is presented to Windows over iSCSI was also set for a 512 sector size:

U:>fsutil fsinfo sectorinfo z:
LogicalBytesPerSector : 512
PhysicalBytesPerSectorForAtomicity : 512
PhysicalBytesPerSectorForPerformance : 512
FileSystemEffectivePhysicalBytesPerSectorForAtomicity : 512
Device Alignment : Unknown
Partition alignment on device : Unknown
Performs Normal Seeks
Trim Not Supported
Not DAX capable
Error: Incorrect function.

No idea what to try next.