The idea behind this module is to allow the user to "browse" within his filmrolls showing him a list of devices and filmrolls. The devices can be in a present or not present state, so different storage space can be used while all the information is present in darktable for its usage and processing.
The user will have an internal HDD with this schema:
~/pictures/2012/01/201201_filmroll1 .../201201_filmroll2 .../201201_filmroll3 ...
And an external NAS with this schema:
/media/NAS/Image_Archive/2009/04/200904_filmroll .../05/200905_filmroll ... .../2010/08/201009_filmroll ... .../2011/09/201103_filmroll ...
Once the user imports all his collection into darktable, the Folder module will show the following schema
[HDD] [Enabled] - 201201_filmroll1 - 201201_filmroll2 - 201201_filmroll3 - ... [NAS] [Enabled] + 2009 + 04 - 200904_filmroll - ... + 05 - 200905_filmroll - ... + ... + 2010 + ...
The upper level shown can be selected by the user, so if the level is 0, a plain list of filmrolls is shown, in level 1, the previous dir in path is shown, and so on until the common ancestor is reached, which is never shown. That will imply that it will never show ~/pictures or /media/NAS/Images_Archive
The devices will be shown as present or not, depending if the physical media in which the images are stored is available or not. That will be made by using libgio:"http://developer.gnome.org/gio/stable/" functions.
Moving a filmroll from [HDD] to [NAS] and back should be available, and that will move all the images in the filmroll to the selected device.