Keep digging down or outward. There's probably a piece of hard stone or copper that it's linked to. It will fall once you mine it.

jump on it and then mine....dont fall too far though :-) some do hover ..not fall as expected... of course you could build a sky home instead :-)

It seems a matter of luck. I do the ones closest to the side first, then the under ones closest to the ground. At some point it will collapse, but it seems you do about 75% first.

you have to dig around and under the node for quite a bit - there are "bounding boxes" that don't necessarily touch the ore node itself, nor are that close to the node sometimes (each box is like larger than a square meter or so?)

after the node is floating freely in all 3 dimensions you have to whack it some more until it breaks - if you freed it from all bounding boxes proper, then the ENTIRE node just bust a nut, lol, super satisfying. if you failed and just a piece of it breaks then well, dig some more around

this mod shows said boxes - the pic in the description gives you an idea of what goes on in the game engine...

https://www.nexusmods.com/valheim/mods/1525