Outside my depth but I'll give it a stab. Identify what data is important, (is the full 42Tb needed?). Can the data be split into easier to handle chunks?

If it is, then I personally do an initial sneakernet to get the fist set of data over. Then mirror different on a regular basis.

This is something we used to deal with regularly on /r/adoption and /r/genealogy

Sounds like an opertunity to build the community here.

Two drives is a good thing. Always have a copy at a remote location and swap it out rather than shuttling the one drive back and forth.

rclone is a good solution. I use it myself. I also just found out about syncthing which is great for syncing with your phone to something to the local network.