tox requires a file system with support for hard links to work
Since tox runs sdist, it requires hard links to work.
That’s a problem if you work in a VM with a guess file system like e.g. VirtualBox which don’t. Thus at the moment, it’s impossible to use tox in such a – thanks to vagrant increasingly common – setup.
There’s a related question on StackOverflow.
The only ad-hoc solution I can come up with is detecting that situation and monkey patch