Error MSB6006: “mt.exe” exited with code 31

Question

I ran into this problem today while migrating a VS2008 solution to VS2010. The problem occurred in either of the following scenarios:

1. Rebuild Solution

Answer 1

Using process monitor and dbgview I discovered msmpeng (Microsoft Security Essentials) was accessing the file, just when mt.exe wanted to have it exclusively. Excluding the development directory solved the problem. It is still a workaround of course.