Why does Visual Studio create a new .vsmdi file?

It appears that the VSMDI problem is a known bug and has been around since VS2005 Team System but it has no clear fix as yet. Another reason to NOT use MS Test.

An MSDN blog details how to run unit tests without VSMDI files.

Assuming that the VSMDI file is under source control, here's a Microsoft Support article about this issue: Multiple vsmdi Files after Running Team Test with VSMDI file under Source Control

Which says:

Someone ran a test while someone else was modifying the vsmdi file. Team Test detects that the VSMDI file is out of sync;therefore, Team Test it makes one and thus you see the incrementing vsmdi files.

And:

Going forward you want to make sure the file is not marked for auto checkout when it is modified. When the current tester has the VSMDI file checked out you, do not want other users to be able to check it out. You want your developers to checkout the file, run a test, and check it back in.

I work around this by always checking out the .vsmdi.
It seems that this only happens when the .vsmdi file is read-only, e.g. not checked-out in a version control system that uses that kind of lock-local-files behavior (Perforce etc).