How to edit Editor.log while Unity is running (on a Mac)

I've been using Unity in Windows for a few years and thanks to a change of job have recently switched to using a Mac.

There's something I'm used to doing in Windows which doesn't seem to be working...

I want to be able to to open Editor.log in a text editor (I use vi FWIW) and change it, while Unity is running, and have Unity continue to write to it.

In the past I've used this to get 'clean' log files recording the results of tests that I'm running in the editor. Deleting all the lines and saving Editor.log just before starting a test, Unity would keep writing to the file and then at the end of the test I could reload the file in the text editor and save it somewhere. Then I could repeat the process and end up with a set of log files each of which is a record of a single test run.

When I try this on the Mac, as soon as I use the text editor to write to Editor.log, Unity stops writing to it.

Does anyone know of a way of doing what I'm after? Is it to do with the OS or have I managed to set Unity up differently?

(I should probably add that the change of job has also seen me move from Unity 3.5 to the latest version)