OS's tested: Server 2016 Standard and upgraded to Server 2019 Standard...issue still persisted.
Storage Spaces (not S2D), reports everything as healthy both in the GUI and PowerShell.
Yet the virtual disk's volume is having tons IO problems and sometimes the volume disappears in File Explorer. How is it that one failing disk is causing this much trouble, and yet Storage Spaces health shows nothing wrong? The application writing to disk is Ubiquiti UniFi Video Server and it will barely function thanks to the IO problem. I am replacing the bad disk to make the problem go away, I'm just wanting to report this issue. Both ReFS and NTFS experienced the behavior.
Pool info:
There are twelve 3TB HDD disks
Capacity: 32.7 TB
Used: 27.0 TB (82.5% under provisioning to allow for 2 disks to fail)
Free: 5.74 TB
Logical and Physical Sector: 4KB
RetireMissingPhysicalDisks Always
3-way mirror, 3 columns, Virtual Disk info:
Allocated: 9.0TB
Interleave: 262144 (256KB)
NumberOfDataCopies: 3
PhysicalDiskRedundancy: 2
Event Viewer:
Event ID 7: The device, \Device\Harddisk6\DR6, has a bad block
CrystalDiskInfo: Shows the disk 6 as bad