Copying Files Drive to Drive Speed

When copying very large files (300+ GB) from one drive to another, I notice that the copying speed varies enormously - 160 MB/s down to 50MB/s and back to 160MB/s again- even dropping to 0 MB/s for short periods.
I suspect that that is a Windows issue but...

If you have a mix of large and small files, these variations are caused by the file system overhead, which affects smaller files more than larger ones. Variations of this magnitude while copying a single large file might be caused by hardware anomalies.