I did some more testing, this time with the drive always plugged into the power pack, and at 100% full battery.
I copied 15 to 20MB NEF (Nikon RAW) files using the web interface, the WiFi button, and the My Cloud App, multiple times.
The last test used 12.38GiB of NEF files in three folders, 703 files in total. The transfer took 30 minutes, at an average of just over 7MiB/s. All the files copied over as the drive never spun down while powered and transfering data. All the files were readable from the MPW. No corruption.
So it looks like the problems I reported above are all directly related to the hard drive spinning down when a transfer is done using battery power alone. That should be fixable in firmware, even if it means a little less battery life. Reliable copying of data from the SD Card to the hard drive is more importation than battery life, as long as there is enough to complete the transfer. Using the speed I measured above, and a real world battery life of five hours, it should be possible to transfer approximately 120GiB of photos. That is probably enough between recharges of the MPW.
