We thought long and hard about this important subject.
The WD 4TB MyPassport Wireless Pro seemed ideal. Until you read the countless reviews of it malfunctioning or packing up completely. The concept is ideal for photographers in the field, but the execution was dreadful from such a top company. In the future, WD will hopefully get the problems corrected, but meanwhile that “ideal” option had to be ruled out.
Instead, we use two “normal” portable hard drives - a Seagate and Toshiba, both with good track records for reliability - making two copies of everything. The chances of both failing together are slim. Using two from the same company (and possibly the same manufacturing batch) would increase risk. A laptop provides the intermediary, as well as providing a great viewing screen. All of this data is eventually backed up even more securely back home.
Whether you’re a professional or serious amateur, caution is essential. “Be prepared” not only works, it’s underestimated as a practical key factor for success.