So, the camera setting (backup to 2nd SD card) makes 2 copies of every photo, one on each SD card. But maybe you are asking how to back up the photos from the SD cards to a usb hard drive? You could use your laptop to do that, or this thing might work (though I have never tried it) https://www.amazon.com/NewQ-Filehub-AC750-Travel-Router/dp/B09T9QKQH7

D7100 SD card settings:

• Overflow: This setting is the default; the camera fills the card in Slot 1 (the top slot) and then switches to the second card.
• Backup: The camera records each picture to both cards. This option gives you some extra security — should one card fail, you have a backup on the other card.
• Raw Slot 1 - JPEG Slot 2: This setting relates to the Image Quality option. If you select one of the Raw (NEF) + JPEG settings, Raw files go on the card in Slot 1 and the JPEG files go on the card in Slot 2.

I use this and it works with any external drive.

https://www.amazon.com/dp/B09T9QKQH7?psc=1&ref=ppx_yo2ov_dt_b_product_details

That drive you post seems pricy to me.

Yeah that would likely work too, BUT I don't think you can create a 'hotspot' from a wifi network, at least not on my laptop (a Macbook Pro). You'd need a device with 2-way broadcasting, like this mobile router I also travel with which unfortunately stopped working on this same trip! 👎

It did work, eventually, via the method I described...

I also travel with a pocket router but annoyingly the hotspot function stopped working this trip so I had to fix it the hard way...