You do understand that by placing the drive names in a TreeView, that's all you have. You must supply code that adds the directory nodes beneath the drive nodes.
I strongly suggest that you only descend one level at a time. For example only add directory nodes immediately beneath a selected drive. If one of those directories is selected, only expand the nodes to include the the selected directory's subdirectories. Otherwise the user will be faced with long wait times.
Also, there is a control to do what you appear to want. It's called
Folder Browser Dialog[
^].