Access Denied on Iesi.Collections.dll

Access Denied on Iesi.Collections.dll

For a project my team has recently started, I've been getting the
following exception when I try to run my newly-created SpecFlow unit
tests:
Could not load file or assembly 'Iesi.Collections, Version=1.0.1.0,
Culture=neutral,
PublicKeyToken=aa95f207798dfdb4' or one of its dependencies. Access is
denied.
Iesi.Collections, Version=1.0.1.0, Culture=neutral,
PublicKeyToken=aa95f207798dfdb4
I (and my team) are stumped on this one, because we've tried the following
things:
Check the folder permissions on the packages directory, where the DLL lives
Ensure that the Iesi.Collections DLL is being copied to bin\Debug on build
Check that the NHibernate settings are sane (they match those of a project
that works for me)
Re-acquired the project from TFS; since I'm the only person this is
happening to, I figured it could just be a case of a corrupted Get. This
had no effect. (Be re-acquire, I mean I closed VS2012, deleted the
existing folder, opened a fresh VS2012 instance, Get > Advanced > Get
Specific Version, with both checkmarks checked.)
Manually re-associating the DLL from the packages directory to the C#
project. No effect.
Because the DLL has a public key token, I know the DLL is signed - so
that's not it, either.
Question: What causes Iesi.Collections.dll to throw an Access Denied
FileLoadException on NHibernate initialization?