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.
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.
Why YSK: If you have digital data that is important, (family photos, crypto keys/wallets...). Back it up and prevent permanently losing it.
3 different places its saved to, (iPhone, ICloud, laptop, Facebook, Google, Camera SD, Flash drive...).
2 different media, I would consider iPhone and iCloud the same. Buying two External Drives of the same type and brand too. Why? Consider losing your Apple account or the drive model fails in a year.
1 off site copy. If all your copies are in your house, a flood, fire, tornado, hurricane, EMP... would lose every thing.
0 time to waste. (My own personal add). Do it now. Procrastination is dangerous and is the biggest regret for when things go sideways.
Something i haven't seen posted here yet, but worth say over and over again.
Murphy's law says that anything that can go wrong will go wrong… but with the 3-2-1 strategy in place, your data always survives.