Plugin with references not working on CRM 2011

Question

I have created a plugin and used a reference to another project (DLL) I have created. The problem is when I try to run the plugin, I get a CRM error saying:

Answer 1

We have had this error several times over the last 6 months and each time, despite me or the other developers swearing blind it would not be the case, it has turned out to be a DLL version problem.

Are you installing the plugin to disk, DB or GAC?

If installing to the GAC, I believe external DLLs go in the GAC as well. If installing to the DB or to disk, I believe DLLs go in the \assembly\bin folder.