Try copying from the C: drive back to the source drive, and from C: to the (original) destination drive. If the slow writes consistently occur only on the one drive, then I would suspect an issue with that drive.
Check the drive's policy settings in Device Manager, and make sure Removal policy is set to "Better performance", and that Write-caching is enabled. Depending on how much RAM you've got, the symptoms could be consistent with the Windows file cache being enabled, but write caching on the drive itself being off.
If the world isn't making sense to you, you're either drinking too much or not drinking enough.