Using the FileList box for custom File Handling Dialogs

I've built a custom dialog with the Drive Combo/Directory List/File List
boxes in it.  Problems occur when there is no media in a removable
drive.  I can trap for this if the user changes TO that drive (from C to
A, no floppy) and tell the user what went wrong.

The problem is if the user HAD media in A, and listed the contents in my
dialog (that works), then pops the floppy and changes the drive to C.
The DriveCombo and Directory List both seem to work correctly, but the
FileList widget throws the exception that the drive (A ??) is not ready.
Why is this (does anyone know)? and does anyone have a workaround for
it?