|My internal IDE hard drive is, in fact, removable. I can install any version of Windows on it and it works (as it should).|
The difference between an internal IDE hard drive and a 'Removable' drive is only the fast connect/disconnect. You don't need to remove the case cover either.
In the context of this thread I believe removable was referring to an external removable drive versus an internally connected but physically external (hanging from the ribbon cable) drive.
An external USB drive is both external and removable.
All drives are removable.