Looking at the code for DiskStore>>basicEntryAt:

I find this code:
"Primitives lookupDirectory: encodedPath filename: encodedBasename)"

What is this "Primitives" not a class? I am suprised there is some code

that I can not find the source for.

Ok, it has something to do with vm primitives, and I know that code like
<primitive: 'primitiveDirectoryEntry' module: 'FilePlugin'>
is in the vmmaker-package for FilePlugin.

BTW it is terrible hard to load vmmaker in a "normal" image. Now I use

the generator.image from pharo http://files.pharo.org/vm/src/.
But even in this image it is not possible to load the vmmaker tool.