Originally posted by Suzaku:

Difficult to mine? Maybe if you're still using the antler pickaxe they start you with. Upgrade to a bronze pick asap to help make further mining easier.

Or ignore the bronze pickaxe, because it's not actually an upgrade. There's nothing that you can mine with a bronze pickaxe that you can't mine with the antler pickaxe, but repairing the bronze pickaxe requires a forge (which practically requires a small base to support it), where the antler pickaxe can be repaired if you have a dozen or so wood to build a workbench with a roof over it. To add insult to injury, the bronze pickaxe requires more stamina to swing than the antler pickaxe does, while only slightly decreasing the number of swings to "pop a voxel" (ie, break a chunk of copper node).

The first pickaxe after the antler pick that you should bother crafting is the iron one.

Originally posted by Suzaku:

As for rare? Nah, it's all over the place in the Black Forest. You just need to get good at spotting it and you'll see a node practically every hundred feet. And note that the majority of a copper node is underground, so dig around the entire outside to reveal it and give yourself a definitive area to work in.

No, they do not respawn, but it shouldn't be an issue with how plentiful they are.

This, on the other hand, I agree with. Wander around in the Black Forest for a few minutes, and you'll find copper deposits all over the place. Once you've found a few and learn what to look for, you'll realize they're all over the place.

You've probably been walking back and forth over a few for hours and never realized it, because other than the muted shine of the copper veins running through them, they look just like every other giant mossy rock; they literally use the same model... but the texture is a dead giveaway, once you learn to recognize it.