I just plotted a WD 4TB My Book Desktop External Hard Drive - USB 3.0 last night on my 280x was getting around 26k nounces per min completed the process in about 10 hours. i might replot the other 4tb i have to try making it go faster... latest drive run in about 46 seconds vs the other 4tb which goes for 74 seconds...

my settings were

with a 4096 stagger size

devices file for gpu plotter

0 0 2048 128 4096

here is the numbers for a 1tb plot
gpuPlotGenerator generate buffer F:\burst\plots\xxxxxxxxxxxxxxxxxxxxxxxx_1_3735936_4096

same settings on my laptop went at 16k/min with a 4gb r9 m375