Well probably this (drivelist_SelectedIndexChanged) was done in a callback, so on the next callback (folderlist_SelectedIndexChanged) page doesn't know about the binded data on drivelist combobox.
This is a normal behavior. The best thing to do is to use a ASPxHiddenField to store the selected item value (of drivelist) on client side, and then in folderlist_SelectedIndexChanged read the hidden field value instead of folderlist.SelectedItems.
You can also perform the following technique:
http://demos.devexpress.com/ASPxEditorsDemos/ASPxComboBox/ClientAPI.aspx[
^]
However, if you search the DevX site:
http://www.devexpress.com/Support/Center/[
^]
You will find other user that got in the same problem and the solution.
Cheers