Slow Write Speed on a Seagate STDT8000100

I set up two Seagate STDT8000100's last week as backup drives and proceeded to copy over files.  One drive is now almost full (345 GB free) while the other has 5.78 TB free.  Write speeds on the almost full drive have remained steady at ~100 MB/s.  However on the other drive they have fallen to ~17 MB/s.  Read speeds on both are at ~150 MB/s.  I'm aware that SMR drives have poor random write speeds but I'm at a loss to explain why the first drive has suddenly dropped in write speed.  I'm copying the same 250 MB and 5 GB files (stored on an internal hard drive) to both drives as a test.