Did you ever come up with a solution for this? I'm interested since it is similar to what I want to do as well.
The wife and I have two camera bodies. Worse case, we have been shooting wildlife all day and have two 128GB or 256GB cards to down load. Downloading each card into an iPad could take more than 1 hour. But to then try to export those images via wifi (even dual band AC) can take hours more. The only thing I can see for improvement is that Apple opens up IOS and put a proper file manage into IOS or you jailbreak you iPad. My iPad Pro 12" should arrive tomorrow and I will do some more experiments with it. I do not plan to jailbreak the new iPad. So for right now our game plan is simply to carry a gracious plenty of 128GB and 256GB of SD cards into the field. If a card goes bad, we could lose all the shots on it. But I have had far better luck with SD cards....that HDs going bad in the field. I trust a Lexar or Sanddisk SD card far more that anyone's HD.
For me the problem with the standalone card readers like the Sanho unit, Nexto unit, or Passport Wireless is the very slow read and write time. They do not read at UHS-II U3 speeds and seem to have slow 5400rpm drives. It would take hours to backup two full SD cards. So I would rather invest in many larger SDXC UHS-II U-3 cards from Lexar or Sanddisk.
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