Seagate 5TB external drive erratic/slow transfer speed, disk activity 100%

[johnciciora]

I have two Seagate 5TB USB3 external hard drives. One of them consistently writes at 120MB/s or better. The other writes jump from 120MB/s to sub 1MB/s and everywhere in between. The erratic drive also stays pegged or close to 100% disk activity, even when no file transfer is occurring.

Troubleshooting:
Have switched USB3 ports and/or cables - no effect
Have tried various source (read) drives - no effect
Have ensured no antivirus, backup, or other programs are reading/writing to the drive
Have reformatted NTFS full - no effect
Have analyzed with CrystalDiskInfo - no SMART errors or other problems
Have tested with CrystalDiskMark - results are terrible
Have verified drive is healthy using Seagate Dashboard Agent

I'm sure I've tried other things too, but can't seem to find the cause. Any suggestions?

Screenshot of task manager and file transfer:
http://imgur.com/iDnCnt4

 

[VirtualLarry]

Are these collections of small files? Or big files like ISO images?

If they are big files, it may be RMA time.