I plan to buy the Samsung Galaxy Note 8 as soon as it’s released. With the previous model now called the Samsung Galaxy Note Fan Edition (FE), you can copy files using USB-OTG and a card reader. This is a good backup solution for the Sony a7rII.
You can get some insanely fast microSD cards now e.g. the . There’s also a 100MB/sec version available in the USA.
When I get the Samsung Galaxy Note 8, I will see what the USB speeds are. If I can find an SD card with a 90MB/sec read speed and a microSD card with a 90MB/sec write speed, I’ll be able to copy files from one to the other and see how it works in real world use. The Samsung Galaxy Note Fan Edition (FE) caps out at about 90MB/sec but that might be because my card at the time could only read/write at 90MB/sec.
They’re getting so fast that it might not even be worth buying a Samsung T3.