Since yesterday, I receive a security exception when I want to run a unit-test from within VS.NET 2008. The error goes like this:

SecurityException: that assembly does not allow partially trusted callers
... 
The assembly that failed was : file:///S:/MyProject/MyAssembly.dll

The S: drive is a mapped drive which points to a physical location on my disk.

What I find very strange, is that this used to work for months previously. I mean, I did this all the time. In order to get this to work, I 've created a new security zone with the caspol utility in order to give this S: network share drive FullTrust. In other words, when I run

caspol -m -lg

I see this (I removed the other zones for the sake of brevity):

1.2.  Zone - Intranet: LocalIntranet
   1.2.1.  All code: Same site Web
   1.2.2.  All code: Same directory FileIO - 'Read, PathDiscovery'
   1.2.3.  Url - file://R:/*: FullTrust
   1.2.4.  Url - file://S:/*: FullTrust
   1.2.5.  Url - file:///S:/*: FullTrust

I've added the 1.2.5 zone just recently because the error that was given, mentionned file:///s:/....

Any ideas ?

Could it be that this has something to do with the installation of VS.NET 2010 or the .NET Framework version 4.0 ?