It can't know in advance if it has free space or not.
1) Other files might get deleted in the process freeing more space.
2) Other files might get copied to the same volume as well claiming space.
3) There may be transparent compression involved meaning that you could copy more data then it seems or that the original data would claim more space in the target volume.
I'm on a PC. I'm copying a 30GB total (VM splitted disk VMware image, a bunch of selected folders containing photos or whatever) on an external disk with only 10GB left. The thing should tell me beforehand if I have enough space, not midway through the process.
Yes I could look it up beforehand, but why make me take three actions (look for free space on device, look for total space required, order to copy) when one would be sufficient? The computer should do the tedious tasks and let me decide.
1) Other files might get deleted in the process freeing more space.
2) Other files might get copied to the same volume as well claiming space.
3) There may be transparent compression involved meaning that you could copy more data then it seems or that the original data would claim more space in the target volume.