Comment 55 for bug 1021375

And it just got really interesting. After fighting this issue for hours, I gave up and resolved to copying by terminal.

By force of habit, I accidently used nautilus once more to copy, after copying a batch of files by terminal, and presto, it works.
Nautilus now copies files without complaining.
I don't understand what happens here.
Some cache of file / partition permissions that went old and was refreshed?