I know I should be diehard and use git in the terminal, but I use a Git client on Mac called Sourcetree to make the whole thing just a bit more sexy.
The problem is a
It is git in general. This answer suggests, it is because of the filelocking. For more info, just search SO for "git samba"
From my experience, operating on a non-bare repo on a network drive is a bad idea.
For best-practises: http://git-scm.com/book/ch4-1.html
So either:
In any case it is recommended that the git repo with the working copy resides on a local disk.