I have a 6-drive mirrored Storage Spaces array in Windows 10, made of 2x 4TB and 4x 3TB drives.
One of the 4TB drives was showing a failure, so I've replaced it and currently have a 7th drive (4TB) attached and added to the pool. Before physically adding the drive, I clicked "Remove" on the worrisome drive through the control panel UI and let it transfer data off that drive, then added the new drive.
According to the UI, that drive has been both "0.00% used" and "Preparing for removal" for weeks now.
Meanwhile, the Storage Spaces pool is warning "Reduced resiliency; check the Physical drives section". The physical drive sections says all the other drives are OK.
Trying to remove the disk through powershell with
Remove-PhysicalDisk warns me that removing it "will cause problems with the fault tolerance capabilities of the [pool]". When I stubbornly try any way, I get the error "One of the physical disks specified could not be removed because it is still in use"
See screenshots for details of everything described above: