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 -