The culprit seems to be "MacFileDirectory class>>isAbsolute:" which 
answers false if no colon is found inside the file name - which is the 
case for a volume name.

To reproduce: Open a file list, select a volume (scroll all the way up 
in the directory pane), and select a file in that volume (not in a 
subdirectory). You may have to buy a Mac first and install Squeak.

- Bert -